Killing a police officer carrying out their lawful duties is a worse crime than killing a civilian and it needs a worse punishment. It's not that the officer's personal life is worth more but the arm of the law they represent is. Killing a police officer is an attack on all of us, on civilization itself, not the officer personally. It calls for immediate and permanent removal from the gene pool. The intention is to deter other criminals from trying their luck. 90% of criminals don't really mind doing time in jail, else one visit would be enough. Jail isn't enough.
This is only true if you subscribe to the philosophy that the people exist to serve the government and not the other way around. Then any attack on a government bureaucrat is much more serious than an attack on the rest of us, because government bureaucrats are superior to the rest of us by virtue of being in the government. If you subscribe to the founding fathers' philosphy, that all men are created equal, then murder is murder, and any murderer deserves the same fate regardless of whom he killed. I just wish the police were as vigorous in pursuing people who kill 711 clerks, etc.