I think Ash adopted Chimchar knowing just how badly Paul treated it. In the flash back in Tears For Fears after Paul let Chimchar go the poor thing said "don't go."
When Chimchar went on its rampage in Chim-Charred it was pretty obvious, from the screen shots, just who Chimchar's rage was directed at. I'm pretty sure that if Ash hadn't tried to calm Chimchar down the poor thing would have killed itself and Paul.
Ash knows how to be one with his pokemon. It's pretty obvious that he doesn't care about just battling his pokemon alone. Ash also cares about his pokemon's emotional well being as well. I don't think Ash would have taken Chimchar on if he didn't care about its well being. Ash cares about bringing out the very best in his Pokemon.
And no matter what Paul might think of Ash's methods of training his pokemon he keeps on trying new things that he thinks might bring out the very best in his pokemon.