There is no in-game policy against “trolling” or “griefing”. Trolling applies to the Forums only and griefing, is a bit nebulous and not a violation of our policies.
“Griefing” is not against policy. If they are killing a quest NPC they have access to, they generally have every right to.
In the end Blizzard decides when and where to enforce the rules, not the player. You can still post in Customer Support if you don’t believe me…