I've read that Thor (and other Asgardians) are forbidden to kill other Asgardians, under threat of being banished.
In S2: Ep12 of Avengers Assemble, Widow asks Thor to promise her that he will "take her out" if she falls under the spell of the infinity stones. Thor says:
"It is forbidden by Odin's law, but I could attempt to banish you to the Dark Realm."
I've never actually heard either way if there was an Asgardian law protecting mortals from being killed. The Wikipedia entries I've read mention the law forbids killing other Asgardians, and there is a comic (which I haven't read) where Thor himself is banished for killing another god. Is this law ever addressed regarding mortals / Midgardians though?