Oh yeah. Speaking as a fan of the classic series, I can't think of a comparable killing. All the Doctors can be callous: the sixth seemed almost sadistic at times, and the seventh famously tricked the Daleks into destroying Skaro. But Solomon was no threat anymore and could easily have been transported out; this was an actual execution, and it went completely unremarked-on.

I can forgive a lot about Who (any Who fan has to), but the one thing that just bugs the hell out of me is how the show keeps chattering on about how cool it is: "Dinosaurs on a spaceship!", "Rory, I'm piloting a spaceship!", "I'm worth any two men!" This episode was far from the worst offender in the new series, but for God's sake I wish everyone would stop attitudinizing.

But for this Doctor to kill a helpless human being—when there were any number of other options available to him—is completely unprecedented. (Remember at the end of last season, when Amy killed Madame Kavorian, and admitted it was something the Doctor would never, ever do?) He's killed before, but always in self-defense or as a last resort: never as revenge or punishment. I'm convinced it's deliberate: we are meant to realize that this is a change in the Doctor's personality, as part of this season's overall arc, and a continuation of the exploration of the Doctor's dark side Moffat has been doing all along. (My gripe with this episode is that no one—including Amy and Rory—remarks on it as unusual.)

I had no issue with the Doctor killing Solomon. I'd have had more of an issue if he didn't. Solomon killed thousands of "people", and in all likelihood it isn't the first time he's done something terrible and wouldn't have been the last. I like that the Doctor usually knows where to draw the line between mercy and punishment (I say "usually," because I had a problem with Tennant sentencing the Family of Blood to eternal torment; I'm OK with killing mass murderers, but torturing them is unnecessary). I also sometimes read super hero comics, and their typical insistence that the heroes must never ever kill the villains, even when the villains are responsible for literally tens of thousands (or more!) deaths is compeltely absurd. There comes a point where one's actions invalidate their right to exist, and Solomon certainly fit that bill quite easily.