Are you familiar with CounterStrike? How does it compare? I guess as long as they're killing aliens -- rather than other human beings, as in CS -- Halo should be okay.
Actually, that was my thoughts. Now, don't go off thinking that Halo2 is not violent or that there is not any frightening imagery. There are some aliens that are pretty gross and scary and some humans do get killed.
Plus, the XBox Live stuff is human vs. human. And, as with any XBox Live game, the language on the headset from the other live players is very foul. We play with the headset disconnected. Now, with Halo2 you can put the live conversation through the speakers - in fact, that is the default. I would recommend you change that if you have kids. We keep it off.