"I mean monsters, not people. I've heard Earth at least has dragons and griffins as mythological creatures."

Sephiroth prefers to maintain a hard distinction between monsters and people, giving himself a clear side to exist on. Even a metaphorical designation of monster, well-earned though it might be, can make him question his already dubious relationship with humanity.

"I'm not certain I'm qualified to speak on social good. It is my understanding that you allow an enemy to live either because it serves some purpose, or because they're no longer a threat to you."

The reason Sephiroth arrived with a yellow ScryWatch is because he hasn't technically done any murder yet, not because he's exactly against it in all circumstances.