The vote in the Missouri case isn't a problem. He hadn't seen it before and therefore couldn't in good conscience reject it without a hearing.
But the pledge to provide a free vote to review death-penalty cases as a matter of courtesy, regardless of his own judgment, is terrible. One, it sets an awful precedent, however seemingly minor. The senators should be given no promises whatsoever. Now that they've been given a promise, it strengthens their ability and inclination to demand more promises of future nominess. Major political blunder by Alito. Two, it ignores the fact that there are four hard-left "justices" on the Court. If Alito is an automatic fifth vote to review death penalty cases, he is making it harder to carry out the death penalty by further entangling it in usually meritless litigation.