SPOILERS: Gun shot at the end

No witnesses is the best reason I can think of.

Not leaving any witnesses is a valid reason all by itself, but I suspect there's more to it... he only saved his mother, after all, only because he was being coerced into it. He was more than willing and eager to let her die otherwise.

I reckon that by that point and having lost his murdering virginity, so to speak, the doctor was already running in full Old Testament mode. I can't blame him.