If they killed you once, it's not griefing, it's just PvP, which is implicit in Open play. Much like playing on a PvP designated server, or entering an area in some games where you are flagged for PvP, by playing in Open you consented to the possibility that you might be attacked by another player. If you don't wish to be attacked by other players, yet still wish to interact with them, I would suggest creating or joining a PvE group. While there may be opportunities for PvP in a PvE group, the likelihood of unwanted attacks by other players is significantly less than in Open.
Had you been chased down and killed multiple times by the same player(s), that could constitute griefing. As it stands at present, you were just unfortunate enough to cross paths with someone looking to take out an easy target.