Normally, he was not THAT guy. But Arrow at this point wasn't a rational actor. His wife was dead, which on top of his grief robbed him of his most reasonable advisor.Yeah makes sense, feel dumb having forgotten that obvious point, still seems at odds with what we know about Harren's nature since he had to have known that the blowback wouldn't have been worth it, would like to see more background than that. Guess we didn't see him in full-blown vengeful grief mode.
By the time he woke up and found out his vengeance didn't make him feel any better? And found out that the Dragon King's son had been killed on top of that?... Well, no wonder he's quite resigned to his fate when the Moonshadow elves showed up.