Question: I am currently writing a story from a murderer's point of view, although I’m struggling to think of a proper motive. I want to make sure the character is grounded with a motive but I also want to keep a mysterious feel to the character. Any advice?
If you want to keep the murderer's motivation mysterious, I suggest you not reveal it to the reader until late in the story.
You may think that will be tricky, if you are writing from the murderer's point of view, but it's not. In fact, this is a typical device in the suspense genre.
For instance, you can have the murderer narrate what he is doing without going into why he is doing it. Often this can be achieved by writing in first person, present tense. You may drop in a few clues, or almost none, until later in the story. Remember, it is up to the killer how much he reveals in his narrative.
In some novels, the first person scenes from the killer's perspective alternate with scenes from someone else's perspective. The killer may even appear in scenes from the other perspective, but not be identified as the same person who narrates the other sections.
Regardless, you should have a sound and specific understanding of the killer's motive even if it is not revealed until near the end. When it is revealed, it should make sense of what has gone before. You can't settle for a vague motive as that will ring less true.
Best of luck.