We are told very little about the exact evils practiced by humans in the antediluvian world (before the flood of Genesis). First, the immorality of the era is described without specifics other than that the masses were totally corrupt to the point of being constantly devoted only to evil and bringing God to regret creating humans (Genesis 6:5-9). Then, Genesis 6:11-13 adds that widespread or extreme violence was prominent. Some potential clues as to particular sins of the time in this category are found in Genesis 9, however. After Noah exits the ark with his family, God condemns the acts of eating blood and murder (in this context, illicit killing of one human by another) in verses 4 and 6 respectively. In the immediate context, he also reveals the moral duty for the remaining humans to increase in number after such a cataclysm (9:1, 7) and more subtly reveals the justice of capital punishment for murder (9:6), which would would make implementing it obligatory.
Many people hear about the sin of murder one way or another, even if the precise boundaries of what constitutes murder are not acknowledged with regularity. Not all killing is murder, for murder is only the illicit killing of a person, which excludes killing in self-defense proportionate to the threat (Exodus 22:2-3), capital punishment for applicable sins (including that authorized in Genesis 9:6 for murder), and some forms of warfare which are the equivalent to self-defense or capital punishment on a larger scale as it is (Deuteronomy 9:4-6, 20:10-15 [1]). Before Genesis 9, basic murder has already been explicitly condemned in Genesis 4. What about eating blood? Unlike simply killing a human, eating blood is always evil, except for something like the incidental ingesting of one's own blood from an internal oral wound. Actively consuming blood, whether of a clean or unclean animal, is a major depravity censured repeatedly throughout the Torah (such as in Leviticus 17, 19, Deuteronomy 12, 15).
This is what Genesis 9:4-6 specifically says about eating blood and murder:
Genesis 9:4-6—"'But you must not eat meat that has its lifeblood still in it. And for your lifeblood I will surely demand an accounting. I will demand an accounting from every animal. And from each human being, too, I will demand an accounting for the life of another human being. Whoever sheds human blood, by humans shall their blood be shed; for in the image of God has God made mankind.'"
Perhaps eating animal flesh with its blood was one of the prominent sins which led to the flood, and perhaps some of this flesh was cut away and consumed while the animals were still living as some insist. Since illicit violence is the broad category of evil said to specifically define the age before the flood in Genesis 6:11-13, eating the flesh of a living creature could certainly be among the sins of violence commonplace among humanity at the time. It is almost inevitable that murder would be among the violent sins directed from one person to another, though of course violence can be far more diverse and far more severe than mere murder. Other violent sins are addressed in the Law, the greatest concentration of these miscellaneous sins receiving attention in Exodus 21.
Irrespective of how normalized it might have been to consume blood with animal meat, eat the flesh of a living animal, or murder other people prior to the flood of Genesis, wicked behaviors towards both humans and animals are condemned at a very broad level through two hyper-specific examples in the account of Genesis 9. The right treatment of both humans and non-human animals (humans are animals too!), while entailing far more than abstaining from murder and eating animal blood according to the precise doctrines of the Bible and their logical ramifications, is really a subcategory of obligatory submission to God, whom all initial living things on Earth were created by (Genesis 1-2) and to whom all things on Earth belong (Psalm 24:1).
Everything I have mentioned thus far other than what some say about eating meat from an animal that is still alive can be demonstrated from logic and texts in the Bible. But one could never tell from the wording of the Bible what depths some people go to in their asinine misinterpretation. Somehow, Rabbinic Judaism, a philosophy based on the contrived traditions of Rabbis, twists such a basic passage as Genesis 9:4-6 to use as a false pillar of a heavily racist form of moral relativism called the Noahide Laws, but the nature of the command in Genesis 9:4 regarding the consumption of blood is bizarrely misunderstood in isolation. Adherents of Rabbinic Judaism might say this command is really about not eating the flesh from a living animal, but no such thing is mentioned. This stance hinges on how cutting apart a living animal and immediately eating its flesh with the blood does involve a violation of Genesis 9:4. However, consuming blood could take many forms, all of which are evil; that is what Genesis 9:4 is about. Eating raw meat with blood from a still-living creature is in fact worse than "merely" eating blood! Because it combines the intrinsic sin of eating blood with the further sin of needlessly inflicting physical harm on a conscious being, though, it is not referenced in the verse at hand.
An obvious distortion of the broader type of sin the verse does condemn, with eating the bloody flesh of a living animal being only one way to eat the blood of a creature, this tradition is yet another example of how Rabbinic philosophy is at odds with the Old Testament and with reason itself. All the same, eating blood is one of the handful of prohibitions specified after Noah and his family depart the ark. There clearly is moral revelation provided to the survivors of the flood in Genesis 9, but it is neither said to be exhaustive nor to be in any way pertinent to the nationality or ancestry of a person as with the idiotic Noahide Laws as proposed by Rabbinic Judaism. Supposedly, God prescribed only seven things to Noah and his descendants after the flood, with nothing else being morally binding on them. Where the Rabbinic distortions of Biblical Judaism err far more gravely than confusing eating blood for eating/removing meat from a living animal is in treating eating blood as one of only seven sins or part of seven categories of sin that are evil when Gentiles engage in them, things like murder, theft, eating blood, blasphemy, and idolatry.
These and the others out of the seven are not the only obligations of Gentiles as according the Rabbinic idea of the "Noahide Laws". Separate from the logical impossibility of cultural/ancestral ethical relativism, Genesis 9:1 and 7 already prescribe a situational obligation to Gentile figures that is completely outside of Rabbinic Judaism's contrived Noahide Laws; it is also true that very few of these seven things are mentioned in Genesis 9. God says nothing to Noah about theft, blasphemy, or most of these seven moral issues that fools say are all that is binding on all people according to Genesis. The Old Testament also never says in Genesis 9 or elsewhere that anything outside of the seven supposed duties/rights to be honored by Gentiles apply only to Jews (including anything having to do with issues like vows, slavery, torture, poverty, widows, and more except as they pertain to the likes of murder or theft), as opposed to humans.
Additionally, the Old Testament gives multiple examples of Gentile sins outside the alleged seven obligations of the Noahide Laws (Genesis 6:11-13, Leviticus 20:6 and 9 with 23, Deuteronomy 18:9-12, Ezekiel 16:49-50, 25:12-17, Amos 2:1-3, and so on). It even separately speaks of general morality, often with no examples, as universally applicable to all people where being a Jew or Gentile makes no difference in one's capacity to act or abstain (such as honoring one's parents or making restitution for theft), because what is obligatory and wicked has to do with the nature of the action or thought, not the identity of the one respectively doing or thinking it (Deuteronomy 9:4-6, Ezekiel 5:5-7, Ecclesiastes 12:14, 2 Kings 21:1-11, etc.). All of this is in accordance with what logic necessitates about ethics if there is good and evil: race is irrelevant to the ethics of an action that by nature can be done to or by anyone.
Accordingly, far more violent deeds are evil for everyone on actual Judeo-Christianity. The unjust violence of humanity that incurred divine wrath as expressed by the flood could have and almost certainly would have entailed far more than just outright murder and eating flesh from a living animal. Of course, eating blood from a dead animal is in no way a violent act on its own, as the violent part would have been the treatment or killing of the animal beforehand. Aside from the unspecified forms of violence in Genesis 6 that clearly do not limit the sins in question to murder, a host of other violent acts are condemned with great specificity. Striking someone with a stone or other object in a quarrel, with the blow not resulting in either the victim's death or permanent injury, is but one example (Exodus 21:18-19). Striking one's father or mother for any reason other than self-defense or participation in Biblically legitimate capital punishment like communal stoning is another (Exodus 21:15). These, too, would logically and Biblically have to be evil when done to or by Gentiles in addition to Jews. As terrible as murder and eating blood are, especially if the blood is consumed while the animal remains alive to seemingly experience pain and terror, a great many other sins of violence would have probably marked the antediluvian epoch as humans long before the formation of Israel grieved God so much that he regretted creating humanity.