IMHO, the world of Angel Beats is like a transition for people who just died and still have regrets left, so to pass on they need to get a "permission" by purging oneselves from them. The Shadow that attacked at the end of the episode might be the result of SSS opposing the world.
P.S.: And I think Hinata really had feelings for Yui, there definitely was an affection between them, so he might have guessed himself what Otonashi was trying to do, and helped him out.
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