In this article it says he has problems with alcohol.
The knife welding "Glenn had moved in with his father, who lives less than a mile away from the Dermonds’ home, after his release from prison in July, Sills said. He has served two stints in prison for battery and aggravated assault."
Though nothing links Ryan Oliver Glenn, 34, to the deaths of Russell and Shirley Dermond, Putnam County Sheriff Howard Sills said there’s several questions still unanswered, namely, his whereabouts on the day of the fatal stabbings.
http://www.ajc.com/news/news/crime-law/knife-wielding-man-arrested-in-neighborhood-where-/nnnzY/
He could have been running around with the knife and stumbled on the Dermonds, he sounds wacky enough for the murders.