[quote=Soul]
That seems a tad unethical and careless. Because of his morals, he basically gave the aliens more time to kill people and cause chaos? Meh.
[/quote]

Yeah its a bit like batman.
If he just killed the joker it would stop him for good but instead he locks him in Arkham to escape and kill again. 
I don't really agree with the stand point but I just thought I would give my take on The Doctors standpoint, I look forward to how the cremator treats the morality that his mentor taught him.