Against it.
I don't agree with the moral arguments against it like "it makes us as bad as the murderers". There are plenty of scum out there who deserve a death penalty.
The main problem is there is always the risk of getting the wrong person. Even with all the latest forensic technology, there is always a chance that a person found guilty isn't guilty, and is later found to be innocent - and innocent people could end up executed. Miscarriages of justice are always a possibility, as there is no such thing as a 100% infallible justice system.
Even if you introduce it only for cases where it is in no doubt at all, there is this risk because once introduced it would end up being used in other cases where there is doubt. Cases like the Woolwich murder where there is no doubt are rare.
And the amount of time and money it costs because of the appeal process is also a big problem.
While I oppose the death penalty for those reasons, I am in favour of stricter sentencing for a lot of crimes. And life sentences should mean life.