This is one lesson I had to learn the hard way. I let a down-on-his-luck ex stay in my spare room about 10 years ago and it was a nightmare. I felt sorry for him cause his kid's mom had kicked him out, he had nowhere to go, he begged and promised that he'd be the perfect roommate, yada yada yada. Turned into the biggest nightmare ever- he wasn't a ham but he turned out to be a secret druggie. He was always stumbling around the house high, making noise, breaking things, abusing my pets, and barging into my room to berate me for all the perceived injustices I'd done him while we were dating. By the time I finally got him to leave, he had completely trashed the room. When I went in there to clean up, I just sunk to the floor & cried. It reeked of smoke, there were cigarette butts everywhere, the beautiful parquet floor was covered in scuffs & burn marks, the sheet set I lent him was stained & ruined beyond salvage, there were bent spoons and beer cans everywhere, holes in the walls, moldy dishes, sticky stains everywhere, etc. So yeah. Never take in the person whom everyone else has kicked out, because there's usually a damn good reason they've run out of places to crash.