No, a police officers life isn't worth more than others. However, they do put themselves in harm's way to better their community and to arrest the people that every day people simply do not want to be around. Yes, they do it willingly. But that doesn't make it any less difficult or heroic when they die.
Take Detective Ellis for example. All reports I've seen say he was investigating an armed robbery. They also say the neighborhood where he was shot is awful. Most people wouldn't want to drive through that neighborhood most likely, let alone knock on a door then blindly walk into a house to search for an armed thug who might be hiding around the corner waiting to kill you. He was doing something to help someone else and gave his life for his community.
I absolutely think we should punish those who are willing to hurt them or kill our police as much as possible, and I have zero problem if murdering a police officer carries a greater penalty than a normal murder.