Verlaine is insane.
When I say this, I don't mean "Wow he's so wacky, he's insane for killing Chuuya's friends cuz who would do that". I mean it in the most clinical sense of the word. There has been a genuine, tangible decline in his mental state from the Verlaine nine years ago to the Verlaine we see in Stormbringer.
Verlaine's goal before being separated from Rimbaud was clear. He was going to raise Chuuya away from his past, not to end his loneliness, but because "I want to save the other me". He wanted to protect Chuuya from going through what he did. He states upfront that he will not give Chuuya to the government because "imagine how it feels to be told you weren't born with God's love, that you are nothing more than a character set someone suddenly came up with. Imagine the depths of a person's heart pierced by those words. It's a pitch-black abyss where the moon can't be seen. There is no hope. There is no salvation." He refuses to let someone else, the other him, grow up with that.
His stated intention when he returns, however, is completely antithetical to this original goal. It is no longer about protecting Chuuya, though he deludes himself that it is, it is about he loneliness and desperation and he is willing to become what he hated to hurt Chuuya into feeling the despair he feels so someone else can experience it with him. He never wanted Chuuya to know what he was, and yet he was the one who became so insistent about Chuuya's humanity that it almost causes Chuuya to fall down the same path. Why was this different? He'd had the trauma with Pan and his humanity, what he didn't have was the isolation.
I want to emphasize how much his goal has shifted to be about his loneliness. I do not think it can be understated how much nine years of total solitude can affect a person's mind. Humans, including artificially created ones, are a social species. Social species experience severe cognitive decline when isolated. Though he'd resented him, he once had Rimbaud. That isolation allowed his resentment and trauma to fester into delusion and obsession and caused his sanity to slip.
Throughout Stormbringer, we see that decline. Particularly in Code 3 and 4. Even the way he talks slowly shifts from cocky and arrogant to desperate and fearful. Fear of himself, of confronting Rimbaud, and even what he did to Chuuya. He is not well, that much is obvious, and I think it's hugely in part to those nine years.
I want to add that Verlaine did feel guilty for what he'd done, and realized that in his search for reprieve of his loneliness and suffering he ended up more alone than anything. Upon realizing this, he begged Chuuya to kill him, which he did not. But still he was dying, and he wanted that outcome. He didn't want to live with what he was or what he did. But Rimbaud forced him to stay alive and live with that guilt, taking away his option to take the easy way out. He had to live with everything he did.
