Certain animals are more aggressive than others. Crocodiles are more aggressive than alligators, zebras are more aggressive than horses, some species of bears are more aggressive than others, etc.
So what environmental influences select for aggressiveness?
Ir is my understanding that the ancestors of chimps and Bonobos were separated by a river. Those on one side had to compete with gorilla’s for food and land while those on the other side did not. The ones who had to compete became chimps while those who didn’t became Bonobos, which are less aggressive and violent.
Robert sapolsky once wrote about a tribe of baboons (I think) where the most aggressive males took over a good supply that apparently had tuberculosis. They all died, and the culture of the tribe changed to become less aggressive with them gone. However I don’t know if that provides info on why one species becomes more aggressive than another.
I’d assume competition for food, land and mates are the big reasons. I know many animals get much more aggressive in mating season
Everyone tells these “just so stories” about evolution focussing on the advantages. Remember every attribute comes with a cost. Bigger brains need more nutrition. Bigger muscles need more nutrition. Bigger size needs more nutrition. periodic droughts or other food shortages can eliminate the neediest - the biggest, strongest, smartest - first. The hobbit pygmies of Indonesia and the miniature elephants of Malta evolved in response to dwindling resources. Assorted horny beasts head-butt to contest for females (like goats, deer, wildebeests, etc. - but rarely do they use those lethal horns to inflict serious carnage on each other - it’s more ritualized.
So the level of aggression is relative to need. Horses and zebras is a bad comparison - domesticated horses have been specifically selected for tameness. Similarly, alligators live in dense swamps and attack small prey, while crocs seem to more often go after very big animals and live in more open rivers where they have to move and attack fast.
Lions run in herds (prides) and run down prey; when not occupied, they lie in a group. I assume less aggression is needed to protect yourself when you’re a dozen lions in a heap, rather than say, a solitary tiger; you have to be more vicious because nobody is helping you get dinner, and any rivals are more likely to attack a solitary cat. Plus, for tigers, pouncing, which seems more aggressive, is their mode of getting dinner; whereas for lions, it’s running it down. Size and group size also matter when chasing food or defending. Aggression is wasted if your usual opponent typically outclasses you. then, retreat or hide is the better option.
I have trouble imagining that one river would provide sufficient divide over the timeline needed for separate species to emerge. During that several thousand years, it would be surprising if at least one breeding pair of gorillas did not make it across the river. More likely, it’s based on valleys or some such ecological niches with decent significant divisions between them - a mountain range or wide swamp or dry plains…
Oceanic whitetip sharks live out in the middle of the ocean, where food sources are few and far between. They have to pursue anything and everything that is, or might be, edible. Whenever they sense something in the water, they investigate it, and usually try to bite it. If it is not edible, they spit it out and move on. If it seems like it might be edible, they become very aggressive. They are a notorious hazard to shipwrecked sailors.
Evolution favors individuals that are good at staying fed and producing offspring that survive to reproduce. “Agressiveness” (something of a vague concept) is one way to succeed, in particular for predators dealing with tough / scarce prey.