User blog comment:Kuopiofi/Saitama - Absolute or Supernatural?/@comment-25893909-20160906164927/@comment-25893909-20160908065928

The reason Saitama couldn't kill either Garou or Boros was because firstly, Boros had a healing factor, which failed to protect him a second time. Secondly, Saitama was holding back, otherwise, both of them would have been mince meat.

Saitama is invulnerable to all kinds of damage, except pain, and even then, that pain is limited to cat claws.

And Saitama is a parody. He's just a comedy character like Bugs Bunny. Nothing can harm them under any circumstance. If anyone can defeat him, it's the Presence (DC), TOAA (Marvel) and Zeno (DBZ).

And if anyone thinks ONE will change that, think again. It would contradict the entire plot. The plot says that Saitama can destroy anything in one single punch, but then we see what would truly be the result if that happened, that's possibly why Saitama holds back everytime. He also wants the fights to be fun.