What is the irony of 'You are trying to kidnap what I have rightfully stolen, and I think it quite ungentlemanly.'" Five, pg. 153 in the princess bride ?

Vizzini tells this to Westley. The irony is of course that Vizzini had kidnapped (stolen, same thing) Princess Buttercup in the first place and is accusing Westley, who is trying to save her, of kidnapping.