I know it's really hard on you, especially as you were already grieving, but you gave Freddie the most selfless gift there is to give, the gift of love, a name, a home, food, comfort, care, and peace.
With his leukemia so advanced, and apparently tumors on top of it all, there was no other choice to make.
As you say, he died with dignity, and he did not suffer. His suffering ends as yours begins.
I'm so sorry.