Problem with Heman is that he is mentally unstable and an addict. Neither makes him stupid, but they make him entirely untrustworthy and they interfere with his ability to think things through. I agree he could have fed Richter a line of bullshit. As for intelligence, we haven't seen much of that in his actions. All he has shown is that he can use his abilities in a really obvious and suspicious way - Richter had all sorts of alarm bells going off on several occasions, Heman barely held his cover for the couple of days he was in the village - to forcefully gain trust and that he is stupid enough to attempt murdering a powerful lord, while ten miles inside said lords lands with no idea of the lay of the land inside or outside said area, nor any way to make a fast escape. He basically set himself up to flee into the unknown on foot with limited supplies while pursued by who knows how many men, mounted sprites and mages. Even if he succeeded in the assassination there was close to 100% certainty that the sprites would track him down, maybe even before he reached the border.