You have to be in a very secure situation to think this way. You also ignore that a war can prevent more problems down the line though often it doesn't. When applying game theory to these situations, depending on how you rig the utility function, you can get any chosen strategy as optimal. So it is more about how you value outcomes and if you are estimating their probabilities correctly as to what is the right decision. By your logic, imprisoning or executing a serial killer isn't OK (let's say in this situation we know they are guilty).

Finally you are completely ignoring competition for resources in your analysis. What makes you think more monkeys has positive utility to individual monkeys? You hope that's true, but until you can speak to them, its going to be hard to know.

These are complex decision and you are acting as if there is always one "correct" answer to every situation. Heck, the trolley problem was conceived of to explain to people like you why your thinking is just plain wrong in some situations.