I did this quest once and the guy in tavern room gave me 5k coin but after i reloaded a save to do again something gone wrong. There is someone already destroyed the cart and when i come back to him he attacked me

This is normal. If you don't burn the cart during the same game day that you accept the quest, he will do it himself. I think turning him in may give more experience than killing him (there should be someone from the city watch near the burning cart).

No strange thing is when i accepted to burn his cart the dialogue changed somehow i cant remember. But i went for the cart immediately after i accepted his firework quest. When i got into the cart someone already burned it

It isn't normal, then, but I don't know what could have caused that.

Lol raze are you a developer of this game? You guys are awesome to create a very masterpiece classic game like divine divinity

I do work for Larian now (support and community manager). I joined the previous Larian forum just after the original retail release of Divine Divinity, and hung around until they hired me (before the Divinity Anthology / Divinity 2 Developer's Cut release).

--- and he was an evil mage in BD and a general in a later game