It's no deterrent to people like Dale Cregan. If he knew he'd hang for doing it, he'd be even more inclined to shoot to kill and make damn sure he killed all the witnesses too. It takes a special kind of vermin to buy and throw grenades.
Bizarre thing to do though, stay in hiding, then deliberately lure the police, shoot and chuck a hand grenade at two officers - and then hand yourself in at the station afterwards. I wonder who actually made the call sending them to that address? There's no excusing anything he's done, and he should go to prison for ever, BUT if someone sent the Police there knowing Cregan was armed, then he or she should also be held accountable.