Yeah I'm not paying other people's unpaid bills, I'd just rather you get into trouble

 For the second time this year I got my old Boston apartment's utilities bill sent in the collections for me. The first time I was scared and annoyed enough to let it pass and just pay it. But no more. It's a really big amount, 550 dollars-ish and I just won't deal with it. It's not fair that whoever it is is bothering me with the issue. So I've given my old roommates a heads-up and asked if the bill was theirs. If it was then I just expect them to pay the money and be done with it, I'll pay the collectors. If not (not them or unwilling to pay), I'll contest the charges, and let the utility company know that I wasn't in the apartment anymore, get this out of my credit score, and let them figure out who to charge. Don't care much if anybody else is in trouble, because bro, it's not my goddamn problem.