This.

Killing people is a part of self-defense; at national scales defensive wars are just self defense.

There's a lot of psychopaths in the world who want to kill you and take your stuff. If you don't have a solid system of defense, war becomes profitable for the aggressor. Not to mention situations like Israel where the aggressors have religious beliefs that demand they exterminate you.

This is exactly why saner libertarians accept that one legitimate purpose for government – and taxation – is military defense. If you don't have a military to defend yourself, eventually someone else will form a government to fund a military to take your stuff.