Not sure about gameplay-wise, but if you don't take gameplay into account, Stefan can kill Karel. For one thing, he's a lion branded (and descended from Soan himself, nonetheless) and for another, he's probably just as skilled as Karel, if not more skilled. Definitely has an edge over Karel (also Vague Katti lol). And I've read something about fighters who let their killer instinct control them will lose in battles.
As for me...I prefer Stefan over Karel, mostly for the same reasons Fireman stated. Plus he has this inherent awesomeness. :o