It would depend on context, though. Suppose a man tries to pick up a woman at a bar. She is offended by what he thinks is a great pick-up line, pours her drink over his head, and storms out. I can easily imagine a sarcastic friend saying to the failed Don Juan "Wow, it looks like you are a real lady-killer!"I don't consider either the first sentence or the reply to be sarcastic or ironic.
One of your options is "Looks like you are a lady killer! Nah, I'm a lady killer." That makes no sense. Did you mean "Nah, I'm not a lady-killer" ?Looks like you are a lady killer! Nah, I'm a lady/men/granny/whoever killer.
When/where was this said?You see? That's what I understand as sarcasm... Someone calls a guy a lady-killer and because he doesn't consider himself as a lady-killer he says Nah, I'm a granny-killer which means he can't really get any girls...
We don't know if a sentence is sarcastic or ironic without context.Thanks for all your replies, so my sentence, which was just made by myself, is completely wrong for interpreting sarcastic and ironic in the first place.