Gentle thinks he's just toying with a starry-eyed young student who wants to become a hero.
What he doesn't realize is that he has just antagonized a soldier, one who has scene the world outside of the Shire, one who knows the sacrifices made to bring about that idyllic world everyone takes for granted, one who he himself has shed blood for.
To Midoriya, Gentle isn't the friendly, well intentioned comedic antagonist Gentle is trying to make himself out to be. To Midoriya, Gentle is no different from Shigaraki, Stain, All For One, and Overhaul. He is a villain who is threatening his peaceful world and must be eliminated, and that's that.
If it wasn't for the villains' [s]plot armor[s] comic book durability*, every single villain Midoriya slammed with OFA would have been turned into a gooey paste.
*I find it hilarious that Midoriya's hits which smash 100-ton robots to smithereens and devastate entire city blocks simultaneously only KO humans, with no severe injuries. It's like the author wanted the hero to both absolutely triumph over his enemies with a clearly visible sign of sheer, unfathomed force, while also being free of the ambiguities of having ever killed anyone.
Next episode: OUCH TIME!.