There was a teenage boy (white skin, possibly had blond hair) who had an extra Y chromosome and had killed his best friend by beating him to death with his bare hands/fists. It may have been a made-for-tv movie (and it may have aired on TBS or USA or possibly even Lifetime). Sometime between 1985 and 1995, I believe.

"Law & Order" Born Bad (TV episode 1993) (http://www.imdb.com/title/tt0629187/)?

That looks like it might be the right thing. All these years I've thought it was a movie. (I only saw part of it.)