Lord Tebbit suggests that the argument against the death penalty was always very thin. So lets have a little look at evidence and history.
What about Derek Bentley? - another cop killer. Is that a thin argument hanging someone who did not carry out the act and who was pardoned 45 years later.
Then if the death sentence had been around the Birmingham Six and Guildford Four would probably have all been hanged as well. But they were all innocent. Lord Denning had said when interviewed that if the Guildford Four had been hanged "They'd probably have hanged the right men". But the criminal law does not depend on probabilities it depends on beyond reasonable doubt.
So I don't see that there is a thin argument at all. The chance that you might execute someone who is innocent is all the argument we need.
The deaths of two unarmed police officers in Manchester yesterday was shocking and a terrible loss. But the actions of the suspect are unusual. Why hand yourself in straight after committing such a crime instead of continuing to run? It is going to be a complex case.