How do you think my cat *might* have died?

Actually, it's nearly impossible to know. See, when cats are hurt they run into shelter away from anybody else. It could have been something in the other side of your neighborhood like a snake, or it could have been an oleander plant. Either way, there is usually nothing you can do. Your best bet is to search what plants and items are poisonous to animals and get rid of them. But none of that matters now, give your pet a good burial, build him a little cross, and move on with your life. I know how it feels to lose a little one.