I think that scene's wonderfully dark and conflicting, which is a sign of good writing. Yes, Buffy comes off as cold and ruthless, and you think "That's not the way the game is played!"

But then you remember (a) what she says is perfectly true, she didn't make any promises to the vampire abut not killing it, even if the vampire assumed she would (b) this vampire apparently makes a habit of preying on vulnerable teenage girls by luring them off to a deserted park at night. He probably wasn't planning to show Renee the scenic landmarks of Tokyo - and how many times had he done that before? How many lives did Buffy save by killing him in the only way possible, since he was immune to normal staking or beheading?

Then it makes you reflect on why we judge this slaying as so much harsher or crueler than all the others, when in fact Buffy kills vampires every single night...