Alan had some kind of future vision where all the heros became dangerous to society. Just looking at their individual strengths alone, it would not be suprising for one of them to easily erase an entire village or 2 with a single attack. I mean, Douglasis a Guardian. A primarily defense and buff focused, supporting class. Not even a primary attacker. And he already can out preform just about every offense oriented class who is just an average person, and by a large amount too.
If he were to become evil, the number of lives loss would be astronomical. They already defeated their greatest opponent, the Demon King. So who else could become their enemy, preventing them from acting only out of self interest? Also, there is definitely someone coordinating with Alan who has quite a bit of influence. Alan just happens to be there at that moment? Not likely given how he was in a different city all together last we saw of him (iirc).
I bet some nobiluty or merchant group is tipping him off to the other hero's movements. They could make a good amount of money if the heros were gone. No one strong enough to interfere with their corrupt schemes, who also has a strong sense for justice means big money.