I am very sorry for your loss.
I did some research on this subject. There have been a couple of animals that have had gastroenteritis (vomiting and diarrhea) from eating beetles/ladybugs. I also found 2 reports of dogs that died with no other explanation other than eating the bugs. I didn't find any reports about cats, but I suspect they would also be susceptible. Most people feel these bugs are non-toxic, but it looks like that might not be true if they eat a large amount of them.
One thing you may want to do is have a necropsy (autopsy) done on your cat to see if there were any other reason for his death. Perhaps there was an underlying heart condition that caused the same symptoms?