Is the monster’s attack on his creator justified? Is his appeal for a mate reasonable?

If we analyze the behavior of the creature from a purely psychological perspective, we would all agree that both situations are expected. 
First, the creature's anger toward his creator is justified. He is a hideous combination of dead body parts extracted by Victor from the most horrid places and put together with one, sole malicious intention: to play God and attempt to bring life into an inanimate body. 
When the moment of "life" actually takes place, Victor's ego gloats at first, but then he has to face the reality that he has created a monster. 
Victor cared so little for this human being he was attempting to create, that he did not even consider the humane aspects that would come with life, such as human emotion, the need for protection, and everything that makes us "people."
Instead, Victor's actions are so narcissistic and selfish that he ends up creating a horrible creature which he rejects horribly, and leaves wondering on its own, fending for itself. Imagine being abandoned as a child and getting abused as a result. That is exactly how the creature felt. This is why he refers to Victor as:

the author at once of my existence and its unspeakable torments

As a result, the creature slowly starts to realize his role in this world: he is an abomination. Tragically for the creature, he seems to have been born with emotions just like every other human being. He felt hurt, abandoned, and abused. All of this amounted to the hatred that the creature develops agains his creator. It is justified.
The appeal for a mate is reasonable, again, in psychological terms. According to Psychology Today, all humans need at least one "stable, caring interaction," whether it is with a romantic interest or not. Hence, someone as isolated as the creature should have wanted some company. It may have been the closest thing to "a caring interaction" he would have found.
Victor does not concede. Even he knows that this would be perpetuating and repeating a highly immoral and somewhat sadistic action. The reality is that the monster is, indeed, an abomination. The insanity of the entire situation needs to come to an end.
Another point of discussion is that the creature may have only requested this partner as one of his many angry attempts to retaliate against Victor. Think about it: if the creature recognized his own hideousness, and valued the beauty of the others, how would he ever wantd to mate with someone who looks just like him?
In all both actions are justified, but one must analyze the rationale behind the behaviors of the creatures under the scope of his psychology as an abominable creature that did not ask to be put in the world. 
https://www.psychologytoday.com/us/blog/in-the-name-love/201403/why-we-all-need-belong-someone