Singleplayer and Multiplayer aren't exactly consistent in this regard. Let me explain:

Singleplayer: Marines lock up CELL soldiers in holding cells and disarm them. Makes sense, and non violent. They were even polite about the whole thing, if you listened to their conversations.

Multiplayer: "DIIIIIEEEEE!!", and "**** you, buddy!". We have gone from a disarming policy to sheer hate. Whats up with that?

In instant action marines fight marines.
I don't think multiplayer is supposed to make sense.