Lost my beloved Cat

I was just wondering if anyone has experienced this? I took my beloved cat
to be declawed because of pressure from my husband.
I took him in and he was due the booster for his Cat leukemia vac. They gave
it to him before or during the surgery. He started to wake up then died.
They told me they thought it was his heart. I am wondering at the wisdom of
giving him that vaccine just before being put to sleep for the surgery.
Could that have been a factor? He was an active indoor cat with a clean bill
of heath the summer before. My heart is broken and my mind is asking why did
this happen. If anyone can give me some clues please help me.

Thank you,
Very SAD