Because when they took a closer look, some kind of a sharp object was seen piercing through their hearts killing them in an instant. They were nothing but the wood splinters from the very punishment poles holding them captive.
And just like that, they died a much quicker death.
At the first glance, it was cruel and terrifying. But from a reasonable perspective, Lilac released them from their suffering, giving them a swift death. She killed them instantaneously so that they wouldn't suffer any longer and feel humiliated by screaming and crying in agony.
This could be considered as a form of forgiveness from Lilac to them. After all, she was a guardian angel. Although, she was now in her human form, the unique soft hearted feelings possessed by the guardian angels were still embedded in her soul and personality.
