Think of the Children!
Abusing his ultimate power, without an iota of thought for any of them, he terminated millions of young lives. Admittedly it was instantaneous - painless - but they were alive and now they’re not: hobbies and projects... gone; desires to impress friends and parents... gone; aspirations... gone. Not just potential lives - these were developed personalities with relationships. Gone.
With one thoughtless act he erased every record of their existence. No history books or grave records note their passing. This is an obliterated generation.
All so that his protagonists could meddle themselves a happier future. Time travel is repugnant.