What does "on kill" mean exactly?

When a sigil procs on a kill (to build a stack) does the character have to merely be participating in a group that kills the mob, or does the character have to strike the killing blow?

So long as you contribute to its death it should count.... I think.

You do not have to get the last hit, kill credit is granted by dealing damage alone.

You just need to be involved iin the kill. Not every mob in the game counts towards "on kill" but the vast majority do, usually anything that you gain experience from or can drop loot or you can rally from will count.

TY. I felt just scoring sufficient hits to get exp from the kill would proc the sigil effect but though to check.

A kill is rewarded for any kill you get experience for. This means some spawns in personal story, minions to monsters and nerfed critters might not count for the sigil proc. (very noticable on stacking sigils like bloodlust)