It is possible to believe in the death penalty, as I do, and also see that the state does not get it wrong. It would take few cases of wrongful death to completely turn the public against the death penalty, thus depriving society the protection derived from it.
Alito may have voted this way, just to make a point to the left that he isn't a "racist." Or it could be that he didn't feel he could get up to speed on the case that quick, so his vote was on the side of caution. Let't wait and see how he votes on the actual case. This was just an vote to hear the case.