All of Moa's closest friends and allies have lost their baseline care for human life. They're top ranking hunters who kill the people they think deserve it, because they were wronged or others were wronged. However, they still align themselves with Moa and do what she says despite her being brand new and unranked, because she hasn't lost her humanity. She helped them even when killing or leaving them to die would've been easier. While she doesn't get rattled by enemies dying, she'd never kill anyone with her own two hands. Every innocent life she fails to save burdens her and deepens her sense of responsibility.
She has a good head on her shoulders and a discerning heart that makes her worthy of being King of the human realm and the leader of the top rankers in the world.
Jihan couldn't do it because he died in a reckless, self-satisfying quest and the only reason he's still around is because Moa took his soul out of respect for his legacy. Eundam couldn't do it because he'd made enemies who kidnapped him and wanted him dead, and Moa broke him out. Seohu couldn't do it because he was already leader of the biggest guilds and he had to focus on saving his younger, brasher sister, Seojin (also kidnapped, who Moa also saved despite Seojin trying to kill her early in the series).
Everyone around Moa is intrinsically selfish and they all gravitate towards her selflessness. It's not like Moa is perfect but she sees them all for who they are and casts no judgement, and when she does judge, it's reserved for those who take innocent lives.
