A lot of people think of “venomous” as being a one-dimensional property like strength or speed that you have to build your way up towards. But really it’s just how this substance your body produces that reacts with another substance in another creature who evolved on a whole other continent to you.

There doesn’t need to be a strong evolutionary imperative to be able to kill a herd of elephants, it’s enough for there to not be a strong disincentive not to produce enough venom to do it.