Grieving over the loss of his mother, a shy boy goes to live in the city with his unstable Uncle, who he witnesses commit a gruesome murder, and then must decide within the next 24 hours to be the the next victim or fight back.

The inciting incident is witnessing the murder so you should begin the logline with that.

When he witnesses his unstable Uncle murder…

You should also combine the two elements into one, the death of the mother and the murder. This will make the story more impactful.

When he witnesses his unstable?uncle brutally murder his mother… (After all the mother’s a gonner?anyway, use her death as motivation)

Now it is just a matter of ‘fight’ or ‘flight’ because characters are pro-active: 24 hours of will I or won’t? I, doesn’t cut it.
He either runs or fights.

If the kid runs, make it hard, have the uncle live in the middle of nowhere.

If the kid fights, then set up a special ability early in the story, one ability he’s good at
which will be useful at a key moment in the story.

Hope that helped, good luck with this!