The big tell here is what didn't happen. Even now, at a point when not even Jesse believes Walt cares about protecting human lives anymore, he still doesn't have the gun he stole from Mike on him when he first approaches the fleeing fixer. It's only after Mike snarls that Walt should have known his place and behaved like the working stiff he's cut out to be that Walt snaps, returning to his car, grabbing the piece and ending Mike's criminal career the hard way. Some part of him knew it would be wrong to simply execute Mike – then the other, angry, bitter, egomaniacal part did it anyway. After the fact, a horrified Walt reacts as though he simply made a terrible mistake, since he could have gotten the information he wanted from Mike elsewhere – but this was worse than a mistake. It was a crime, against both Mike and himself. And Walt knows it.