I usually like ruthless badasses, but I guess I haven't played enough of the first game (actually I fell asleep too when my friend was playing), so I've not had much of a chance to see Altair in action.
As for Ezio, I think I can believe he would be a ruthless killer having lost his father and brother(s)...was it brothers? Yeah, I think so. And his mom is a mute, etc... Hell, if someone killed my family and I lived in a time where I could get away with it, I'd go after the sons-a-bitches who took them from me - without hesitation.
I guess that's what made it easier for me to believe. Who knows what the deal would be with Ezio if he wasn't faced with that situation.
I don't think that everyone who kills people (guilty people who deserve to die, for that matter) has to be a cold, ruthless, heartless bastid.