The fate and the future of all three of them was in Gabriel's hands. However, looking at their pitiful state, even Gabriel didn't feel like killing them.
The Prince kept begging Gabriel to let him live, even if he meant becoming a slave. As for Shia, she remained with her eyes closed, waiting for Gabriel's sword to take her life.
Gabriel knew that it was true that they hadn't done anything wrong to him. The only problem was the potential future threat. Then again, could these three really increase his future threats more than it already was? Moreover, he was upset that the Arecia Clan even killed the children, all because they were scared of the future threat, even though the children weren't an immediate threat to him.
He couldn't help but find himself in the same situation as the Alecton clan in the past, having to decide if he needed to kill innocent people to remove potential troubles from roots or let them live.