Professor Gast is just as bad as Hojo, if not worse. I despise him more than Hojo because he doesn't acknowledge the atrocities he causes, but Hojo does. Hojo relishes in the suffering; he's unapologetic.
It's incredibly suspicious that one of the two leading men in the Jenova project ended up marrying the last living Ancient and having a child after studying their kind for years. Gast also forms enough of a bond with Sephiroth for him to remember him fondly, too. Gast stuck his finger in every Cetra pie he could find.
It could have been the guilt of his actions that made him choose to form close relationships with whom he believed to be Cetra but either way, his time on the planet after Shin-Ra didn't last very long. Definitely not long enough to be redeemed, anyway.
