Although Valefor's malevolent aura seemed to have awakened the Fire Fiends' intelligence, it was only an illusion. He had simply awakened their fear of him with his overwhelming slaughter energy.
History may have been erased and changed, but the experience of endless slaughter has been preserved in Valefor.
He had simply killed far too many beings.
Originally, such an oppressive slaughter aura should have been counter-effective on the Fire Fiends due to the wrongful deaths of their past lives that turned them into vengeful spirits of fire.
However, Valefor's slaughter aura also contained his rage against injustice and wrongdoings done to him.
As such, the Fire Fiends felt both fear and belonging, as if they were kindred spirits.
If Valefor hadn't felt pity for their tragic experience, he wouldn't have spared any merciful words.
He would have simply killed them all.