Albus knew he had to punish him, but Greengrass was still his brother-in-law and he did not want to upset Calypso. Also, he could understand him if he had resentment towards him: his sister had seriously risked her life just to be bound to him. Albus himself had had second thoughts after taking the Unbreakable Vow, not because he did not love his partner but because at the time nothing suggested that they would survive. He looked at his girlfriend, who was still alive, just like him, and felt grateful to have her.