Sigh, Ken really screwed up. He basically went from a 100% guaranteed win to very slim. There was absolutely no reason to vote for Matty even if Corrine's idol was true.
If Corrine's idol was fake, then Corrine would be voted out. Back then Ken, Crystal, and Sugar were a tight alliance and could have went to the end and voted out Bob, Matty, and Susie.
If Corrine's idol was true, then Matty would be voted out, however this would not even matter in the end since the alliance of Ken, Crystal, Susie, and Sugar still outnumber Corrine and Bob's alliance. They could of just voted Bob and then Corrine out. Afterwards Ken and Crystal can choose to align with Susie or Sugar at the final 3.
And for the reason of Ken voting for Matty for jury votes was just ********, Corrine's alliance hated Sugar, Susie, and Crystal more than Ken, simple as that.