Ritual murder = we kill because it's our custom; e.g. captured warriors could get killed to celebrate victory.
Human sacrifice (a form of ritual murder) = we make an offering and we expect certain rewards. E.g. we sacrifice 100 citizens because we believe we need to meet a quota and otherwise the sun won't go up.
I'm pretty sure in a Venn diagram the "ritual killing" circle would include human sacrifices.
Ritual killings have elements not related to the act of killing, serial ritual killings demonstrate a repeated pattern of ritual .. it's killing with obsessive complusive and|or psycho sexual flourishes.
I would guess that a sacrifice is made to please some Other with powers mere mortals don't possess, like a deity or a spirit, and a ritual would be a more general term for a spiritual tradition without necessarily pleading to a higher power. But what do I know.
Ritual murder = we kill because it's our custom; e.g. captured warriors could get killed to celebrate victory.
Human sacrifice (a form of ritual murder) = we make an offering and we expect certain rewards. E.g. we sacrifice 100 citizens because we believe we need to meet a quota and otherwise the sun won't go up.