Mike Ehrmantraut (Season 5, Episode 7: "Say My Name")
The murder of Mike, the grandfatherly fixer who reluctantly helped Walt set up his nascent drug empire, is a study in contradictions. It's Walt's single nastiest, most vindictive, most pointless kill – he shoots Mike in the gut because the man pissed him off, pretty much. There's no justification in terms of protecting someone or something important, and Walt himself even tells Mike before he dies that he's realized he could have gotten the information he was after some other way. But the death itself is almost serene, with Mike slumping over quietly after sitting and staring at a sun-dappled river for his final moments. Granted, he had to growl "Shut the fuck up and let me die in peace" to earn that last half-minute of rest.