Came home to my cat dead

Hello everyone I am heartbroken.  Milo was my first cat. I had adopted him from a friend who could not keep him upon moving in w a significant other who had other cats.  According to the vet they used he was stressed by the other cats and had begun to lose weight and display other unhealthy signs. I happily took Milo into our cat free home and made it a cat home! He was never very playful but did earl up to us and became affectionate over time ( we adopted him about ten months ago) last month he became lethargic. And we noticed blood in his eye. We took him to the vet and after several years and several hundred dollars couldn't figure out what was wrong.  After treating him w antibiotics and eye drops his eye improved but not by much however he resumed his normal eating and social behavior and exhibited no sign that he was in pain.  I was working on coming home the money to perform an ultrasound and determine of maybe there was a tumor causing the problem. Well I came home from work tonight to find him dead in a pile of brown puke. I am just looking for any insight into what may have caused this and if it was an issue that he had always had. He had always been under weight but the vet never mentioned that ad a concern. 

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