judging griefing should go by a situation-to-situation basis. you cant make a final judgement about it because it varies from game to game, match to match, and person to person.
look at world of warcraft, 2 level 80s could PvP. if one dies, they are forced to respawn with reduced health. the winner of the fight could be waiting for the respawn only to strike again. and if he kills the unfortunate player again, he is still getting honor. he could sit there for hours and keep doing it if he really wanted to.
i dont think this is griefing. but someone else might. my arguement here is that hes not abusing any game mechanics, he is jumping on an advantage in order to gain points, and further his own development.
if this same 80 in the same game were to do this to a level 20 however, hes not gaining anything, and its griefing.
recently, i was playing Resident Evil 5 online in a coop match. the person i was playing with sounded like he was 8-12 years old, screaming through the headset in a high pitched voice at anything and everything. i was hosting the game, and he would get upset every time i picked up gold or treasure before him (anyone who has played knows its shared in online play). he said i have all this upgraded gear, and that i didnt need it. i told him to calm down, explained that its shared, and to just play, or i would be kicking him out of my game.
he continued screaming everytime i picked anything up, so i had enough, and decided to irritate him to give him something to scream about. several times he ran ahead of me, and when an enemy grabbed him, i had the choice to shoot the enemy and save him, or let him die. obviously, i let him die on several occasions. the round ended horrible, with bad rankings because of his high death count, and he was screaming at me the whole time.
is this griefing? i dont consider it to be. it was his actions that led me to this, and he could have just left my game at any time. some might say it was my form of "retiribution", others might say that i was 100% griefing.
griefing is a grey area of gaming. i dont think we can ever truly get a clear black and white answer on it because its never going to be the same.