What was the reason for the other player to kill that person if all they had was a "rock" as you said before? No reason at all, they didn't even have equipment to steal, so yes, I call that griefing. In this type of game, if you are killing another player simply because they are in view on your screen, that equates to griefing.
You can call it that but that's not griefing. Griefing is intentionally harassing other players using the game mechanics in unintended ways and it doesn't always result in death. Players don't need a reason to kill other players in a game with PvP combat because the game isn't bound by moralistic rules unless expressly written into the game code.
I don't see how a dev can include both PvP and coop while also enforcing some arbitrary set of rules to protect people from perceived unfair deaths in a survival game. The former seems to go against the spirit of the latter. Devs would be better off just making a separate coop survival game to serve that market or have entirely separate game modes. I think that's already happening with games like 7 Days to Die so I think those players will be served anyway.