If I kill the Legion guys and free Benny, will I be able to see him again? I killed him in the arena but regret it a little, I kinda liked him.

You liked that SOB? Weird. I was so pissed at what he did to me that after I talked him into meeting me in the Presidential Suite without his bodyguards I went all medieval on his bee-hind with Chopper. Gave me a great feeling of job satisfaction, yuknowutimean? :fallout:

To answer your question, if you help him escape he basically vanishes from the game.