For the sake of the completion of the Main Task, Allen dared not create a bad reputation for himself in this world. Because of that, he couldn't kill those he wanted to kill without providing any righteous reason or something like that.
The Heretic Gods wouldn't sit still if he ran rampant in this world. If he made an enemy towards everyone in this world, he would be labeled as either the demon king or an evil god.
With that outcome, those Heretic Gods wouldn't miss the opportunity to promote their religion by lending their aid in killing an evil god.
Allen could use others, but he wouldn't let others use him. Naturally, he wouldn't allow the Heretic Gods to use him.
As the Supreme Hidden Boss, it was his job to use others and treat them as his chess piece, not the other way around
