It does have it in the rulebook
In the NHL, an "aggressor" in a fight is defined as the player who continues to engage in a fight after it has become clear that the opponent is unwilling or unable to continue, or is in a defenseless position. This can involve throwing punches, continuing grappling, or otherwise attempting to inflict punishment.
46.2 Aggressor – The aggressor in an altercation shall be the player who continues to throw punches in an attempt to inflict punishment on his opponent who is in a defenseless position or who is an unwilling combatant.
A player must be deemed the aggressor when he has clearly won the fight but he continues throwing and landing punches in a further attempt to inflict punishment and/or injury on his opponent who is no longer in a position to defend himself.
A player who is deemed to be the aggressor of an altercation shall be assessed a major penalty for fighting and a game misconduct.
A player who is deemed to be the aggressor of an altercation will have this recorded as an aggressor of an altercation for statistical and suspension purposes.
A player who is deemed to be both the instigator and aggressor of an altercation shall be assessed an instigating minor penalty, a major penalty for fighting, a ten-minute misconduct (instigator) and a game misconduct penalty (aggressor).
There are rules in fighting, remember boxing and UFC also have refs too. When refs get in between, there is an unwritten rule that you don't throw punches but there is a written rule when the player is laying down on the ice and defenseless. So I guess Trenin broke both rules. The fact that you even question this shows me that you dont know what the rules are. In fact Trenin was kicked out of the game but its hilarious Canucks didn't even get a powerplay out of it. Just pathetic reffing by NHL imo.