So sorry that happened to your roo. Crummy thing to see.
I have heard many, many sad tales like yours from folks who use chicken wire on their runs. I am really sorry it's even called "chicken wire," because it's very misleading ~ that stuff is useless at keeping out predators, and many folks are not aware of it until it's too late. We used 1/2" hardware cloth; it's more expensive, but it is pred-proof against anything except maybe bears.
Hope this experience won't discourage you from keeping chickens. Despite this unfortunate event, there is much pleasure to be gained from the little darlings. Take care!