The code that puts a first-person-shooter avatar and reticule on top of random uploaded videos doesn't appear to be publicly available. That's probably a good thing because it makes everything much more stomach-churning.
YouTube Shooter is the work of intermedia artist Kent Sheely, who's also swapped the old-school Nintendo Zapper for actual guns in a bunch of iconic movie scenes. The 2013 art project is practically ancient in terms of internet time but I just ran across it today and haven't been able to stop looking at it. The fact that the overlay obscures the button that closes ad pop-ups is an unintended consequence that makes the work even more darkly humorous and self-conscious.
Seeing how it turns the most blasé video sequences into too-long sequences of dread—who's gonna get shot? when? now? now?!—it's understandable why Sheely hasn't unleashed the code that makes this overlay possible on the world.
There's a morbid curiosity about what various YouTube videos would look like in YouTube Shooter mode but it'd also make recordings of birthday/wedding/bbqs into major bummers.