I believe Sollecito admitted in his book that he had experimented with hard drugs. I suppose we can say that Guede and Sollecito, who both lived in Perugia, had a similar vice as well: hard drugs. It's surprising that their paths had never crossed given that they didn't live far from each other, and Sollecito had been studying in Perugia for four years - granted Guede was still in high school for two of those years and he had worked North of Milan after completing high school.
It's true that Guede was trouble for his adoptive parents, where, as a teenager he was truant, lazy and just wanted to play basketball.