Guilt and Anger - my dog died after getting neutered?

My dog was 12 yrs old. I brought him in to get him groomed(which is attached to vets)before I collected him they asked me to go in and see the vet. She said he was in pain and 'had' to have the operation to get neutered. He had lumps around his anus which bled sometimes or he scraped his bum along the ground. I knew that sometimes he was in pain and this was on my mind to bring him in. When she said this the lump in my throat. But I felt she was the vet and she knew best. So I said id think it over. Thing is I asked all my family n they were not really interested(family split up) although my dad said to leave him, but dad was sometimes old school mean so didnt really listen to him. I decided to go with it, they assured me they'd be watching him the whole time on a drip giving him oxygen etc..this didnt happen...My instingt told me he wasnt able for such an op, wasnt being walked enough and too old, however i felt id no1 to ask and felt silly asking people not everyone values animals(how v silly of me and this makes me so mad with myself) the night before the op i still wasnt sure - i told dad he was gonna decide when he came home from the pub - but unfortuately my brother turned up drunk banging on the door - he wasnt allowed in the house as he was too abusive etc..he wrapped on the door for hours until eventually we had to let him in - there was a row, my dog went into my bed for safety and peace - he trusted me. the next morning after v little sleep - i ordered my brother to bring the dog in for his app with the vet. I reluctantly went to work - v nervous ringing my brother to say im not sure about this but at like 10am that morning - they called me to say he was dead!!!!! I fel like the ground had been removed from underneath me - the worst thing that ever happened to me and iv had alot of bad things happen to me but i loved this dog to bits. he was walking and still eating he may have been in pain sometimes but no need to kill him. WHY DID THE VET HAVE TO REMOVE HIS TESTICLES SHOULD SHE NOT HAVE KNOW HE WAS TOO OLD FOR THIS! HE WAS STILL FRESHLY GROOMED - I COULDNT BELIEVE HE WAS LIFELESS - WHY DIDNT THEY JUST TAKE OUT THE LUMPS N LET HIM LIVE A BIT MORE??? THEY SAID HE WOKE UP AFTER THE OP THEY PUT HIM IN CAGE AND CAME IN 15MINS LATER AND HE WAS DEAD - MY HEART IS BROKE FOR HIM THERE ON HIS OWN..WHERE WAS THE OXYGEN AND DRIP THEY TOLD ME ABOUT? THE ONE THING I LOVED MOST IN THIS WORLD I LET SLIP THROUGH MY FINGERS. THE POOR THING! WHY DID THE VET DO THIS WHY DIDNT I RESEARCH MORE - BIG BIG MISTAKE - anyones opinions would be greatly appreciated