Your RBTA killed a mantis in a 75 gallon reef? Wow. I wish I would get that lucky with my RBTA... Oh.... And sorry for your loss.
As an aside, how did the RBTA come in contact with the mantis for long enough to kill it? They seem to have very hard shells, and move very fast? Did it wander in at night (do you have moon lights?). I have a clown in my tank and it doesn't let anything come near the RBTA.