It's called
Cruel Mercy which are acts of punishment and benevolence. These adventurers who betrayed the "weak" Dion and left him to die believed their strength permitted them to kill. Now, they are the weak ones, they are the ones who would have died if not for Dion, and they know it. They must suffer knowing the trash they tried to kill saved their lives. And even with Yuki not killing them in retribution, leaving their team would weaken them further. It's also likely this story of their crimes will spread, even just as gossip, and they will be known as a group of betrayers of their own party. No one will trust them, no one will work with them. They will be alone, effectively banished by the masses for their crimes.
Yuki, for her part, will think little of them ever again, but they will remember this always. They will suffer for their cruelty and cowerdice, and never know respect again.