It was stated in the press in the earliest situations after he left TCU and was with Seattle. Same thing for Manziel after he kept doing his garbage and his bipolar disorder info finally came out. It's very commonplace among singers, actors and athletes who achieve incredible success then suddenly spiral out of control. Even stars who didn't crash & burn have written about the struggle they've had for years that was kept hidden until exposed in their tell-all memoirs. It is very "tricky" dealing with it and Boykin's background probably didn't provide the best of the best professional help in his youth, and certainly after he no longer had the rules & regulations of whatever TCU impressed upon him, his freedom held too much avoid, and as so many have said, the meds just held him down, or back, or whatever.
If you check on most of these guys who go into churches, schools, last week's jewish family's home & etc., to commit these horrible crimes out of the blue, the articles in the following days which interviews the perpetrators family or friends seem to say that he's not a bad guy, has mental problems, gets into trouble when he goes off his meds & just needs help, because he doesn't mean to do what he did. And so many times, "bipolar" appears in the sharing of their knowledge of his situation.