Explain Like I'm Five

Hippos. They hate almost everything, but sometimes some of them hate each other more.

Some spiders tend to eat their partners, just because they're murderous, not hungry.

Different types of snakes tend to be quite racist and can't stand one another.

Koalas. They can be very aggressive about their leaves and will try to kick each other off trees.

Some sharks just don't give a shit on what they bite. Maybe because they're mostly blind.

Kangaroos will drown you, your kids, your pets and other kangaroos. Because fuck this, fuck that and fuck you. Kangaroos are psychos. Don't mess with them.

PS: Some of these examples might have specific circumstances or are plainly wrong. It happens.

Unsurprisingly, its Chimpanzees. I've once read about a case where researchers studied a certain group of chimps, and observed that from time to time a young would disappear from the group, never to be seen again. Turns out an older female was luring them away from the group and murdered them. Apparently, there is a non-interference rule in wildlife research, so this went on until the group simply died out, because every single young fell victim to the serial killer.

cats seemingly kill birds and other wildlife for fun

(i love cats, before anyone gets upset)

I had no idea and decided to search around. While this may not directly answer your question, this article was a good read:

https://crimereads.com/the-most-murderous-mammals-adventures-from-the-dark-side-of-science/

Orcas. Orcas are menaces lol

Insert murder cats graphic from oatmeal

Have you heard of Argentine ants?

Wikipedia: https://en.m.wikipedia.org/wiki/Argentine_ant

Radio lab episode: https://radiolab.org/podcast/argentine-invasion

Domestic tomcats frequently kill every kitten they sire. The theory that I hold is that the tomcats murder kittens to both prevent competition and to prepare the queen (the female cat - and yes, they are referred to as queens) for mating sooner.