Why does Vizzini say he resents the man in black?

From the text:

The man in black froze.
"Better," Vizzini nodded.
No sound now beneath the moonlight.
"I understand completely what you are trying to do," the Sicilian said finally, "and I want it quite clear that I resent your behavior. You are trying to kidnap what I have rightfully stolen, and I think it quite ungentlemanly."