Here's a hypothetical scenario:
Imagine I hypothetically went back into time and prevented someone I don't like from being born. Not by killing his parents or forcing them to get an abortion of course (because that would definitely be murder), and not even by preventing them from getting together. I would simply become a friend of his father in the past, and invite him for a sleepover or a party or something on the night the person whose birth I am trying to prevent was conceived in the original timeline. As such, he will be conceived the next day, or some completely other day. By then, the sperm cell that originally conceived him will be a lot older (as sperm cells only live about 5 days) and slower, or dead. So some other sperm cell will fertilize his mother's egg cell, and someone with different DNA (like a brother or sister of the original person) would be born, and the person I am trying to remove would never be born at all.
Would that be murder?