Well, look at Kael and Vashj. You have the option of destroying thier bases, but it would be wise to do it anyway. So that you can use all your attackable units to fight Illidan's squad. But its hard to say if Illidan is really "dead". Cause if Kael and Vashj survived, they can drag his ass back and recover him (but who knows). However, I find it dumb he fell for an obvious attack.