These guys are space vikings, protect the weak, can out-drink the Emperor himself (and still fight the morning after), and love tearing heretics a new one WITH THEIR BARE F*CKING HANDS.
They are also immune to the effects of the Warp; instead of becoming warp-influenced traitors, they become feral Wulfen (okay, it's not much better, but at least the Warp still collectively sh*ts itself when trying to deal with those crazies).
Mincemaker said: So....when influenced by the warp, they became werewolf marines? That is definitely worse!
It's a gradual metamorphosis as Space Wolves get older. In this case, it's not a Chaotic mutation, but a defensive reaction in their gene-seed when their DNA is flooded by the powers of Chaos. Wulfen are actually more resistant to corruption than the standard Astartes, making them rather effective against the forces of Chaos; they just can't be controlled. You just have to know when to let them go nuts; that's what the 13th Company is for.