Depends upon what said "mercy" entails. I strongly feel that setting him free would be not only a poor decision but dangerous as well. Considering that he killed someone (even though that someone was his abuser), in conjunction with the history of chronic sexual and physical abuse, he presents a very high risk picture as it pertains to public safety. Furthermore, research indicates that men who are sexually abused by their mothers are at increased risk for sexual aggression and violence. Sadly, I am admittedly unsure that he can be "fixed" at this point. For example, his initial lie to LE along with his chronological age, imho, makes a strong case for stunted development (most adults and even teens would not consider such a tale even remotely believable much less attempt to put such forth).
In all, and imho, this is a troubling case all the way around. Esp in a society that focuses on retribution as opposed to public safety. The best I think we, as a society, can do is to consider this case a lesson wrt the plight of sexually abused boys, and instead of making discompassionate remarks, vulgar jokes, and/or sensationalizing their horrid abuse, recognize and try to get help for the victims before they move to the next stage. Bc once they cross the line (what I refer to as the point of no return) and become the victimizer, it is, imho, too late.