Having sided with mr house till the end... I never got to kill or meet caeser in battle... just the legate.

so what became of him?

is there no way to and fro the legion fort/camp.. there is a gate by the trader which doesnt seem to open.

Killing Caesar isn't part of the main quest, and Mr. House won't be mad that you kill him. You can waltz into his tent and kill him inside his camp. I did http://fallout.wikia.com/wiki/The_Fort the entrance to his tent is near the arena. I don't recall exactly as I haven't played it since it came out.

You can kill him.
The game however is set up to make it a challenge however, and for House it's implied that you should not kill him.
That said go nuts, the game makes many suggestions that you can just go against with no trouble.

As to the outcome, untill another game is released only speculation can be made.