Vengeance is sweet. Revenge is a dish best served cold. The best revenge is a life well lived. Revenge—well, you get the point: people have a lot of things to say about revenge. In fact, it's a human emotion as strong and passionate as any other, like, say, love. Or friendship. In Frankenstein, revenge has an emotional resonance way stronger than Victor's half-hearted protests that he really does love Elizabeth, honestly. (Think about it: on his wedding night, who does he spend most of his time thinking about? Uh-huh.) The monster may think he has no connection to the world, but revenge gives him a continued link to Victor. It may be distorted, but it's still a way of forming human bonds.
Although revenge forms a very destructive type of bond between the monster and Victor, it ultimately becomes their shared link to humanity and gives them a reason to live.
Victor's desire to avenge William's death ultimately brings about the deaths of Henry, Elizabeth, and his father. Victor is therefore morally responsible for these tragic events.