I think an understated aspect of Genesis' pre-Degradation personality is that, while he was jealous of Sephiroth and a hothead in battle, he was also very lenient with him. He surprisingly did not blame Sephiroth for his injury or his subsequent illness. Sephiroth himself confirms that things were alright between him and Genesis afterwards, at least verbally. Angeal was the one who lectured him. And it really wasn't fully Sephiroth's fault. Genesis injured HIMSELF. And I think Genesis knew that. I think it was definitely a huge blow to his ego, but it's more like a reminder that he couldn't measure up to Sephiroth's level, NOT that Sephiroth was to blame for his own misfortunes.
I don't think Genesis' initial intention was to hurt Sephiroth. I think he left solely because he was bitter at SHINRA and sought a cure for himself. He was distracted by his illness, not looking to get back at Sephiroth until they were well and truly enemies on opposing sides. Genesis DID start to get really petty and nasty later on. And he did continuously alert Sephiroth to his jealousy and desire for glory. But he still did not blame Sephiroth for what happened. His hostility towards Sephiroth carries remnants of their past rivalry, but is practical in nature. Sephiroth is still on Shinra's leash. And then, later on in his own mind, Genesis attempts to emotionally knock Sephiroth down a few pegs in order to get something from him. Horrible strategy. But it makes sense if he wanted to shock Sephiroth into saving his life. And it's also extremely likely Genesis was being influenced by his degradation. The man is DYING. And is pretty mentally and emotionally damaged.
Point being, I think there are SOME layers to Genesis' feelings towards Sephiroth. It isn't all hate and bile and envy. I don't think Genesis viewed Sephiroth as his greatest enemy. Definitely someone he viewed as a rival, but there's still that thing with the apples. It was still his greatest wish to share them with Seph, and to become his equal. I think he lashed out in plenty of areas, many of which were personal. I think he was pretty awful to Sephiroth at the reactor and this was building up for a long time. But his arch nemesis is SHINRA as a whole. He didn't blame Sephiroth for everything. Even when Sephiroth blamed himself.



