what happens if i kill caesar,ncr leaders,legate,house in the last level?

You hope you're doing Yes Man's quest path

Try it an let us all know the results

I did this.... and I installed Yes Man. Would be interesting to see if you can somehow finangle not installing Yes Man, getting to the last battle and then destroying everyone... but I bet the devs thought that through....

But in my playthrough I killed: Caesar, House, Oliver, Legate.

J

But did not assassinate the President of the NCR? You missed one.

In my first playthrough, I did Caesar's quests up to destroying the Brotherhood. I decided I'd rather tryout Yes Man then (I never saw him in Benny's suite, never talked to Benny about it). Doing the first couple of Wild Card Quests made me hostile with everyone else. I installed him and he said, "Wow! You really destroyed all those securitrons huh? It's...fine, no problem at all!" Let's review: Vilified by the NCR, killed Mr. House, couldn't finish Yes Man's quests and couldn't finish Caesar's questchain (killed everyone in the bunker, BEFORE taking off the collar. Fast-traveled to The Fort, and saw my head explode. I thought if I killed 'em all the explosive would deactivate.)

If you're going to kill all those people, you may as well http://fallout.wikia.com/wiki/Meat_of_Champions