Technically, isn't toxic behaviour when someone does things that promotes other players to perpetuate an action, attitude or likewise the same behaviour to the detriment of a community?
That's what I understand as "being toxic". An example, someone caps-locking in a dungeon after dying, because a tank failed to hold aggro.
Is the anger justified? Sure, maybe, depends on the context. If the context is, the player is a sprout, doesn't know the dungeon, the player in question was pulling more and the healer didn't have the gear or skill to keep the tank up - that's where a player would be considered toxic. The act of exploding in rage doesn't improve the tanks ability, doesn't make the healer "play the class" better and it certainly doesn't make the dungeon go any faster. And before you know it, this new sprout is going to take away your behaviour as the norm, and repeat it one day further down the line towards another new, or less skillful, player
Even if this "ideal" situation isn't considered, it is the act of perpetuating (knowingly or no) these behaviours that makes something toxic. It's like a venom, spreading and killing the host (which in this case is the community).