For some the difference was (historically) why killing slaves wasn't murder though now our definitions have changed and it would be considered murder, setting aside the comparative morality of slavery. Some people are vegetarians or vegans because they regard the killing of animals for food as murder.
I think the distinction is sentience. If a creature is sentient then it's considered murder, but if it has no significant level of intelligence (as defined by ourselves) then the argument becomes whether the killing is necessary or unnecessary, rather than whether it is murder. Japanese whalers, anyone?

Mick