In Mary Shelley's Frankenstein, why does the creature kill Henry Clerval?
Frankenstein has foolishly made a promise to create a companion for the monster. However, he soon gets cold feet over the idea. He's already starting to realize the full moral implications of creating the original monster; the last thing he wants to do now is create another one. A further consideration is that, if the monster has a mate, then together they'll be able to create a new race on their own without any involvement on Frankenstein's part. Not only does that present a future for the planet too horrible to contemplate, it also undermines Victor's original plan to have a race of creatures who'd worship him like a god, owing their existence to him and him alone.
So Frankenstein reneges on his promise to the monster. Not surprisingly, the creature is furious and seeks revenge. The monster still wants a mate, and he knows that Frankenstein is the only man capable of doing so. It wouldn't, therefore, make much sense to kill him. So he's not going to attack Victor directly, at least, not yet; he's going to get at him through the people he cares about. That's why he threatens to turn up on Victor's wedding night if his demands aren't met. And it also explains why he kills Henry Clerval. He's punishing Frankenstein for going back on his promise to create a companion. In doing so, he's adding to the crushing burden of guilt which is already weighing heavily on Frankenstein's shoulders.
Interesting question! In the book Frankenstein by Mary Shelley, Victor Frankenstein’s creature kills Clerval. Although Clerval never personally hurts or injures the creature before this incident (even though he does call the creature a “Hideous monster”), the creature believes that he must seek revenge against Victor’s for his wrongdoings. As a result, Clerval experiences the creature’s wrath.
In the story, Clerval is an innocent young man who is a close friend of Victor's. As a result, the creature murders Clerval to seek revenge for the pain that Victor causes the creature (such as the pain from being created and rejected by Victor). As the creature states:
“Frankenstein! you belong then to my enemy—to him towards whom I have sworn eternal revenge; you shall be my first victim.”
Thus, once the creature discovers that young Clerval is related to his creator (they're friends), the creature murders Clerval. Although the creature does not initially plan on committing this murder, his desire for revenge incites this crime.