Not only was there no law establishing Riker killing his clone being illegal, but his and Pulaski's dna was stolen from them. They were assaulted with a weapon and subjected to a procedure they said no to earlier in the episode. It would be like telling a woman who was raped and was pregnant due to that rape that she had no choice but to carry that baby to term. It's her body, her choice. Same applies to Riker and Pulaski's clone situation.