I think that we're supposed to think that it is Steven. Obviously we are all going to dislike him, as he is the one who insisted that Lizzy be kicked out of the tower. He was also really determined not to go on guard duty while Pippin was there, which could be interpreted as him knowing that Pippin would recognise him when he saw him. However, if he turned out to be the rat I don't think that it would have the same emotional impact - so far we have not seen anything good about his character. It would be much more devastating for the rat to be someone that the other characters have trusted.
Kalani seems to be a more credible rat to me. Kalani was a pain in the ass at first, but he has redeemed himself in recent episodes. I don't remember Kalani and Pippin meeting face-to-face; Kalani was the one who originally saw him on the video monitors, and he was suspicious of him, he told the others to be careful because Pippin had a gun. I don't think Kalani actually went down to meet him at the gate with Angel and Datu though. If Steven really is a red herring, then it is probably also supposed to throw us off that Kalani and Datu went out on a mission just before the alarm went off; though the timing must have been incredibly close because when Michael was telling Angel to look for holes in people's stories, he specifically said to include Datu and Kalani in the grilling.
Also, if he is a Maller then maybe that is why Skittles recognised him... Skittles had said that he thought Angel and Kalani were 'the bad people' before he rescued them, and Kalani brushed him off quite brusquely when Skittles asked if he knew him.
It could also be Riley, however; when Michael and Angel rushed downstairs to check on Hope, they bumped into her in the stairwell. They asked if she had seen anyone, and she said no. She was one of the people with no alibi. Maybe she got drunk to prepare herself for killing in cold blood? Michael and Angel don't seem to have considered her as an option, maybe because they are close to her and trust her? Wouldn't it make it all the more devastating if she turned out to be the traitor, though, rather than it being a random character we haven't met before? However, she has been in the story since the first episode, which means she couldn't have lived with the Mallers after the outbreak happened. Perhaps she was in jail with them before everything happened, and has some sort of loyalty to them. We don't know anything about her history.
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