Trust. No relationship can survive without it.
An assassin can't trust anyone. He certainly can't trust anyone in a multiplayer mode. He tosses furtive glances about, ingesting and dismissing most of what he sees without thinking. But when his eyes fall upon a potential threat, those eyes narrow and he becomes a shadow. That not-quite-conspicuous person over there is your friend, or rather, he was your friend… Things can really get out of hand, which is why Ubisoft has launched a website dedicated to rebuilding trust amongst those who once trusted each other, but now harbor nothing but feelings of hatred and revenge. It's only normal.
At the site, you will find helpful tips on Respecting Others' Feelings, Maintaining an Open Dialogue, Keeping Eyes to Yourself, Playing With Others and Healing Old Wounds, along with special Tools (you can even send a gift to your friend). Lessons range widely and of course, everything starts with "Admitting You Have A Problem." The question of, "is it still friendly fire if you use a knife?" should have an obvious answer if you're serious about reforming.
And remember, listen . "It's not me crouching up and down over your face after I kill you…it's my character crouching up and down over your face after I kill you." And how does that make the ex-friend feel? "Makes me feel like just plain killing me isn't good enough for you." Very true, Mr. Victim. Very true. But now, we can do something about it, if you're willing to work on the problem. 'nodding sagely'