It's interesting that you point to the animal kingdom. Like you, I can't think of any animal that is violent when all of its needs are met. However, many animals can only continue to exist by eating other animals. So while it's true such animals are violent out of a scarcity of resources, there is no way that they can ever reach a state in which the scarcity will not return.

Humans can experience psychological scarcity, as you point out, but we can also choose to allow a need to go unfulfilled for the sake of some perceived good. The example that comes to mind is holding one's pee on a road trip until a convenient rest stop is available. Excretion is one of the most fundamental needs, but we can willingly forego even that need for a period of time, even to the point of, say, skipping multiple rest stops to get some extra miles on the road before stopping. With psychological needs, especially, I think humans are capable of relinquishing their claims on such needs should they choose to do so. This is where moral evil comes in. If we can choose contrary to some drive that would result in some natural evil, then failing to make that choice would typically be a moral evil, that is, it's not a necessary result of causality, like natural evil is.

Regarding your last question, I believe there is never a justification for premeditated murder. It is always wrong.