(For the sake of argument, "Evil" being whatever hampers the human ability to survive, thrive, and be happy, otherwise life, liberty and the pursuit of lolis.)
The urge to dominate and punish and torture in the worst possible way is a primitive part of all of us, and it is even worse than in animals, since animals don't seek to avenge or commit retribution ten fold or whatever. All they do is react for the sake of survival instinct. Humans dwell on past wrongs and stew on thoughts of revenge. I know many famous quotes that refer to this, though I can't think of any off the cuff atm.
Also, you have to figure.. whenever something like the Old Testament was written... 4,000 BC? 6,000 years ago? Man (Humanity) was not nearly as civilized as it has been in more recent centuries (occasional backslides notwithstanding). For one thing, consequences had to be severe for anyone to pay them any mind. Also, I suspect there were practical life-manual type things being expressed. For instance, I suspect people probably got sick or died from eating shellfish in certain areas.. shellfish being bottom feeders and feeding on whatever fecal matter and trash lay on the ocean floor (an easy way to dispose of garbage in those days). Also, populations routinely got wiped out by famine, disease, war, floods, etc.. Being fruitful and multiplying was a dire prerogative in those days. Whereas today some say there are TOO MANY people on the planet, in those days there was often an existential fear of a tribe or city being completely wiped out, and proper behavior was critical to survival.
I dunno.. just random thoughts. I've never claimed a strong belief in religion, but I do see the practical issues it has addressed across time. I'm open to alternatives that make people choose to behave in ways beneficial to a civilized contented society. Not really seeing any take root yet, tbh. Lay em on me, anybody.