I'm sorry to hear that he passed away, but don't beat yourself up. Many of us feed layer feed to all of out chickens. It is kind of a trade-off--either your hens don't get enough calcium in their diets, or the roosters and non-layers get more than they need. I have never lost a rooster in 6 years from feeding layer feed. I used to use Flock Raiser, and still would if I had chicks around, but my flock gets all layer. Your rooster could have died of so many different problems, such as a heart ailment, respiratory, or any number of problems. State vets and poultry labs do a good job with necropsies for a set fee, and a couple of states do them free. Refrigerate and ship or take the body as soon as possible for necropsy. Here is a list of state vets and labs:
http://www.usaha.org/Portals/6/StateAnimalHealthOfficials.pdf
http://www.metzerfarms.com/PoultryLabs.cfm
http://www.usaha.org/Portals/6/StateAnimalHealthOfficials.pdf
http://www.metzerfarms.com/PoultryLabs.cfm