He might have been more compassionate raised outside of a labratory setting. Certainly Angeal and Genesis, raised by actual decent people, either felt it or could mimic it better than Sephiroth could. But with his upbringing in Hojo's hands..

"Right before the fox turned up and I ... felt compelled to go with it," he says slowly, measuring his words carefully. "My team had decided to ignore their orders and turn back to save an enemy civilian, abandoning their duties in a lethally dangerous situation for nothing more than the illogical whims of emotions." He doesn't sound like he approved. A child's disapproval is a heavy thing. "I went with them."

Sephiroth turns a little to study Zack for a long moment, that little frown still in place. "But in fifteen years, I won't. Not even for someone called my friend? Did he not matter as much to me as they do?" He sounds irritated by it, but that's not really irritation. It's something worse. He trusted his team, and when they chose to ditch everything and risk it all for one person, he chose to do so with them. What kind of person would not do even that for a friend?