Why do we believe in necessary evils?
If something is evil, it should never be necessary. Anything that necessitates evil must itself be evil, and why are we doing things that are evil? Let's look at international affairs; people talk about how war is a necessary evil, and that while war may be a bad thing, a thing to avoid if possible, it is necessary that we occasionally do this bad thing. The presumption is that the necessity somehow forgives the evil, makes it permissible.
Why do we think like this? When we live in an international system that NECESSITATES evil, which in itself must make it evil, surely we should consider changing the system? Rather than just saying "this is how the world is, it is idealist to think otherwise, we must accept that war (a necessary evil) is a part of the world." Why not change the world, to quote one of my least favourite Clapton songs? If our current systems necessitates and permits evil, an evil that we try to avoid, surely we must change the system.
Necessary evil is still evil, and we should be questioning what necessitates it; even if to do so is considered idealistic.