It's all that "scion of light" and other such nonsense that drove Haochen to a foolishly greedy plan like this. He started to believe it, that he's the one man who will solve the whole demon invasion problem that has existed for thousands of years. He and he alone (with his squad) must deal with it. Furthermore, he feels a terrible urge to do it as soon as possible. He's not even 9th step yet, but he attacks the demon capital and wants to kill the second most important demon god and destroy the corresponding demon god pillar. That's nothing but greed, even if it's not greed born out of malicious intent. It's not like the human defences had yet collapsed, in fact a few demon gods had been slain instead.
Is this Haochen's fault? No, I don't think so. He's not even 20 years old. It's his father, grandfather, and the whole knights temple's fault. The pumped him full of the "chosen one" bs. It started with his idiot old man and his "you aren't allowed to meet your family until you reach the 7th step" shit. That created the permanent emotion of urgency and impatience within Haochen. It certainly didn't help he always made progress through exceptions and special treatment, partially due to his scion of light status and partially because he landed first places in competitions. That gave him a false sense of supremacy. Ultimately it led him to bite far more than he can chew. He has a bunch of impressive achievements, yes, but his level of experience is still paper-thin. He was way out of his league challenging the top 3 demon gods directly.