In PMD I found in a dungeon a Kecleon who was standing in front of some items. I took them then he went all rage mode and almost killed me (and I had 999 HP). I did recruit lots of Kecleon tho. How/why did this happen and why is it so strong?
This is just how Kecleon has always been in every PMD game. If you try to take an item without paying, he will fight you for it, with more strength than any Pokémon, and even if you manage defeat him, more Kecleon will appear.
Here is where you can learn more about Kecleon's Shop and all the gimmicks: https://bulbapedia.bulbagarden.net/wiki/Kecleon_Shop
May this quench your curiosity.