Glad you're enjoying multiplayer!
Try joining a clan (you can tell because the gamertag will have an acronym in brackets) if you find a group of players whose play style you identify with.
Re: random knives, it happens. Some people will knife you while climbing to finish the challenge, while others will do it because it's so tempting. Other times people will do it thinking it's their pursuer.
Look up on Youtube how to knife stun. It will give you a bit of an edge if you master it.
And try not to fall into the temptation of running around like a 6 year old with ADD. Yes, you'll finish a match with 13 kills, but those silent/incognito bonuses are worth much more.
I remember winning a Deathmatch where somebody was about to overtake me in the last 10 seconds and there was only a 50 point difference between us. He started a chase with his target and I threw knives at him to screw him out of winning. Dude yelled at me for most of the next match, it was great
GT: romertremor http://i.imgur.com/jcW29.gif
Pawn - Donovan/Sorcerer
Joining a clan is definitely a great way to better yourself in terms of overall skill. Just the opportunity to play with/against skilled players frequently is invaluable.
Your madness! How it shines like a beacon!