I think I figured it out!
Ok guys! I think I figured out who the rat is. I think my argument is pretty solid. Here it comes...
KALANI!
We first meet him at the Arena when Datu was captured by the zombies. Samantha says that she doesn't know who Kalani is and that he was there before she was brought there.
Kalani fakes an illness so the zombies don't pick him. Very deceitful!
Then, back at the tower during the party, I don't remember where Kalani was when everyone was getting drunk. I haven't had the chance to listen to it again before posting this, but even if he was in the scene, he wasn't there for very long. I think it was during the party that Kalani contacts his old pals, the Mallers.
Fast forward to when Angel and Kalani are getting the MRE's. Skittles says that he knows him from somewhere, but Kalani denies knowing him. If Kalani was really at the tower, wouldn't Kalani have known who Skittles was? And Skittles has already proven to have a good memory, having remembered the socks he received from Angel.
I think Kalani infiltrated the other Tower and then allowed the zombies in to kill everyone off. After that, the Mallers would have come in and taken all of the supplies or even the building as a new shelter. Obviously, it didn't exactly go as planned. However, the Mallers have a second shot with this new tower.
It may be a little disorganized, but that is my explanation of why Kalani is the rat. What do you think?