Preventing genocide next door is defense, not offense. Just like in Ukraine now. The most Offensive thing here is your defense of genocidal wars of aggression.
The problem here is that when you give a defensive alliance a mandate to defend entities that are not covered by it's mandate it becomes a de-facto offensive alliance.
There's a question of semantics here of course. We're contrasting aggressive and defensive military action. I don't see how characterising military action in defence of others as aggressive is a useful or meaningful characterisation. It's defensive, just not defensive of yourself.
Also let's look at the objectives. The intention in Kosovo wasn't to invade Serbia but to end ethnic cleansing. The fact it took force to achieve that was a means to and end, not an end in itself. Without the ethnic cleansing there would have been no military action, so it wasn't about territory or aggression.
If the way you choose to prevent genocide is by marching an army into a foreign country, that's called "offense". Your argument is sophistical.