I don't think they do. As Gondolinian pointed out above, the Doctor has been known to kill the fat man on the trolley (sort of--I can think of situations where he lets them go to their doom, but not where he personally pulls the trigger). But the Doctor has also been known to refuse to kill the fat man on the trolley (as in Kill the Moon). I don't think the writers agree on anything more than "he does something weird or extreme that people like myself wouldn't do", and they're not consistent in which weird or extreme things those are.
I don´t think so, but yeah, who knows? That´s the beauty of this show, it is weird. Btw, when I said the writers, what I really meant was the people who are currently working on the script and the ones who are involved in cross-season plots.
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