Question about doing right thing for my dog

Yesterday my beagle wasn't able to support himself and was fine the day before. When he got to the vet they said his temp was below normal and he had bad blood work. We choose t do an ultrasound and it showed a mass & his liver was bleeding and only option was surgery. He was 12 and I felt this might be worse. So we brought him home and our vet came i the evening and put him to sleep. Now i am questioning myself and feel this incredible loss. How do you know if it was right?