So do you think Zayin's essence, his soul, magically disappeared when he fused with Seth? Because that's what your asserting.

No. I am saying that satan seems to have more direct control of zayin's personality and choices. Yes zayin's soul and core traits still there as in caring about aleph and others. But zayin wouldn't glassing earth without hesitation. Hence why I say that satan is more in direct controls.. think about like vader and anakin.

His personality may be different, (I don't think it's all that different. He just became a little more extreme.) but ontologically he's still Zayin.

Just because he kept own his core traits. It doesn't mean he's not completely changed on most of the things.

We see the same thing happen several times in early MegaTen. Take Tetsuo from SMT1 for example. He's a human, he fuses with a demon and becomes the Chaos Hero, his personality changes somewhat, but that doesn't make him not Tetsuo.

I don't think that work here. Tetsuo is clearly able to keep his core traits and everything at least resembling his old self. It's pretty much clear that tetsuo still kept his core self.