Easier in a physical sense, but hurting a kid, even a super-SOLDIER one... that'd be rough. What's also rough is Sephiroth's response. Zack had had a hard time fighting Angeal, Sephiroth, even Genesis to an extent, but he'd done it because they'd forced his hand and not fighting them would mean they could continue hurting others. He couldn't allow that. But Sephiroth... he's talking about putting a friend down like a dog, so they no longer suffer. He's not sure how he feels about that, beyond the visceral sensation of wrong.

"I can't speak for future-you, but... I know you guys had a falling out. It doesn't mean that you weren't friends before Genesis went rogue. Just means that you had limits for what you were willing to put up with, you know? He's the one who abandoned everyone."

Genesis could have asked for help, instead of running off on his own. Zack doesn't know a whole lot about what went on behind the scenes, but he'd heard enough. Sephiroth tried to help. It's not his fault no one let him do it, or bothered to explain what the problem even was.