He doesnt want to fight the Demon lord because he knows then and that is understandable.
Also he was fired because his boss (Bashubaza) was always compared and told to follow Dariel advice, this of course given him a hell of a inferiority complex and he fired him just so he could prove that he didnt need Dariel, as much people here are oh so mature his reaction was kinda expected and had a long time brewing, Dariel is still liked in the Demon Army as you can see by most reactions towards him, he doesnt want to go back because he found a place in the village.
As for the Heroes ... I mean, subversions should not became the rule, I mean yes ... outside Bashubaza they dont seem terrible but then again, who knows ... Bashubaza was making a mythil bomb so you shouldnt just take the ones we saw as example, after all we cannot just apply "humans-bad, demons-god" because subversions.
I do doubt he will join the heroes because the title is "The Slow Second Life".