The creature doesn't murder Victor because it wants to make Victor suffer much like it has been forced to suffer. Victor created the monster only to abandon it immediately after it is brought to life. Because of his hideous features, the monster is shunned by everyone it comes in contact with and must face the world alone. The innocent creature is enraged because he is treated badly based on how he looks and sounds (he had yet to acquire language skills). In order for Victor to have a taste of what it is like to be alone as well as to make him suffer for his ill treatment, the creature begins killing off members of Victor's family.
Even though the creature was bent on extracting revenge from Victor, he still views Victor as his creator and father. It even weeps over Victor's dead body on Walton's boat before going out into the harsh winds of the North Pole to die.
Another reason that the creature keeps Victor alive is that it wants Victor to create a mate for him so that he would not be lonely in this world. Had Victor done so, the creature would have ended his rampage against the Frankenstein family. When Victor "denies [the creature its] wedding night", the monster to vows to be with Victor on his.
In addition, the creature chooses not to murder Victor, but to make Victor dependent on the creature. Victor is all the creature has in the world. There is no female companion since Victor changed his mind, ripped her up, and dumped her in the lake. There is no loving family (Victor and the DeLacey's abandon him), and there is no quiet assimilation into human society (everyone runs, screams, throws things at him, reacts badly to his size, shape, and physical ugliness). By tormenting Victor and killing all those Victor loves, the creature is making Victor like himself. At this point, Victor also has no one in the world except the creature upon whom he has sworn vengeance. This is why the creature leaves notes to antagonize Victor, hints as to where he is going, and food for Victor once they reach the icy climate. He does not want Victor's frail human body to give out on him...the creature is dependent on this dysfunctional family unit he has created by killing off Victor's loved ones.
Consequently, when Victor dies on the ship, Robert Walton witnesses the creature's true despair. The creature tells Robert that he will kill himself since he no longer has any reason to live. As readers, we have no reason not to believe that the creature will do as he says since he has proven throughout the novel to be very dependable.
The monster doesn’t kill Victor because he is his creator, and in that sense his father. He also wants to keep Victor alive so that will he create a mate for him, telling Victor that doing so will make him good: “my virtues will necessarily arise when I live in communion with an equal” (142). Finally, when the monster realizes that Victor will not create a mate for him, he understands the best revenge is to torment him forever, for to make his life a hell would be worse punishment than releasing him from life. He tells Victor, “you live, and my power is complete” (198), and he is right, for Victor’s life then becomes consumed by tracking down the creature, a journey which ends with his death.