I keep thinking that, once found, he's going to have some kind of violent background. It may or may not have resulted in any arrests, but I bet there will people in his life who could come forward and give examples of things that he's done that were (for lack of better word), scary. JP hadn't killed anyone before Mr. and Mrs. Closs, but there were still some eye-raising anecdotes about him. Not as many as I expected them to find, but some. With BG I could see someone who's gotten in trouble with an employer for losing their temper, perhaps some DV issues with a former partner, maybe an assault charge that may have been later dropped. Something made him feel like he could do this. I don't think whatever that was has gone away.
Sorry, long answer. TL
R He may not have any murders in his past, but I bet there is some anecdotal evidence of violence and I believe he could kill again.