Luffy isn't going to kill anybody ever. It's not in his nature, not should it be. He beat down Doflamingo, the Navy has him in custody, and he's out of the picture in Dressrosa. That's all that matters. Even if the government wants to execute him, they literally just took him prisoner, and they weren't going to shoot him on the spot. But they almost certainly won't. For one, he no doubt has massive dirt on all sorts of powerful individuals, and maybe he'll feel chatty about them at some point. Much more importantly...he's a World Noble. A disgraced, exiled World Noble, but still. (And a former Warlord to boot.) That's not the type of blood the government is going to willingly spill. At least not publicly, anyway.
And why shouldn't Doffy get one more word in? He's a great villain with swag for days.