As has been stated, it's been reported that they shared a key to the place, and it's a deadbolt lock, so Karena couldn't have locked the door unless she also took the key, which would leave Faith without a way to lock the door if she left in the morning before Karena returned.
Is leaving the door unlocked irresponsible? Sure, but if you've ever lived in a roommate situation as a young person in a college setting, it sounds 100% natural to me... people coming and going all the time, partying and drinking, last person to leave locks up... it may not be a good thing, but it's not at all suspicious or out of the ordinary IMO.
I lived in a house that was converted to student apartments in my student days, and we had a communal living room/kitchen. Nobody EVER locked the front door. But each bedroom had its own lock (a real lock, not your typical bathroom door style lock). I'd walk through and there would be some stranger sitting on the couch watching TV, and I'd just say hey and go on to the kitchen, lol. They were always friends or guests of one of the other 4 people who lived there.
College living is... different. Everybody is everywhere, and crashing on other peoples' couches, etc. More than once I woke up on a stranger's couch to find that everyone else had left and I was alone in a strange apartment, and had to let myself out. There's something fun about living like that too, until someone is murdered of course.
