Here’s the scenario - there are two friends, Rick and Jessie, Jessie’s got himself a girl and Rick wants to make her his.
So rather than being funny and being cool with the lines he’s gonna travel back in time, instead of feeling dirty when they start talking cute he’ll prevent Jessie’s parents from meeting so the point is probably moot.
When he comes back to the future something’s changed, it ain’t hard to define, Jessie no longer exists because Rick’s been traveling in time. Assuming all he did was prevent Jessie’s parents meeting via innocuous means (distracting them with a guitar riff or something), is this tantamount to murder?
Either Rick is morally responsible for all changes to the timeline, or he is morally responsible for none of them. If he is to be charged with the murder of Jesse, then he must be declared the cause of all children created by the new couplings made by Jesse’s former parents, the murderer of all the children not created when those new couplings were made instead of the original ones before the timeline was screwed with, and so on and so on and so on.
What happens is morally wrong if, and only if, you can show that the world after the change is worse off and needs to be changed back.
I’d argue that every murder, if successful, results in somebody not existing. Whether you’re causing somebody to die by shooting them or causing them to not be born by time travel, you’re intentionally turning somebody who exists into somebody who does not exist. The morality seems equivalent to me.
Rick is responsible for the murder of every person who ended up not being born because of his changes but he is also responsible for the gift of life to all the new people who were born, that weren’t before, because of his changes.
So Rick is now God and needs to be worshipped as such.
“All hail Rick! All Hail Rick! Oh Rick can you see? By the dawn’s early light!”
Of course it’s murder. His intent was to remove the guy from existence, and he performed an action designed to make this happen.
Many worlds theory doesn’t work, since he traveled back and found that the guy didn’t exist, and apparently did not find another him who didn’t go back in time. So we’re clearly on Back to the Future rules.