1. It isn't part of the Noahic covenant. We find the passage on capital punishment in Genesis 9:6, but the covenant part begins in 9:8.
2. There are some other directives that are given with capital punishment, such as not eating meat with blood in it. Those of us who eat rare to medium steaks break this command, but few Christians think this still applies today. Maybe it does. But then there are some other aspects, like God giving all animals for Noah and his family to eat. God rescinded that in the Mosaic Law when certain animals were restricted, but then opened that back up with Peter's vision. So obviously these things are moveable and not necessarily set.
3. Keeping in mind that we intuitively believe some of the other items mentioned in Genesis 9 are moveable, we seem to have clear teachings from the New Testament that Christians are to love enemies, do no harm, etc. If Christ can usher in a new freedom for food, it seems he can do the same for sinners - even murderers.
4. God doesn't say here that capital punishment is only to be used for murder. We read that into the text. God's argument for capital punishment is that human life is so valuable, if someone kills another, then they themselves should be killed. We see this same sort of notion with the Ark of the Covenant when Uzzah tries to save it from falling. While Uzzah even had a good intent, God killed him for marring what is holy. We see something somewhat similar in Numbers 35. While God differentiates between murder and manslaughter, he does make allowance for a blood avenger to kill one who commits manslaughter. Even though manslaughter is not malicious, God seems to recognize that a human life was taken and can allow for retribution. So if you want to take God's valuing of human life and his allowance for retribution here in Genesis, I think you have to take a look at the language and recognize all that might entail.
5. Even if this command in Genesis 9 stands, it doesn't mean Christians should enact it. If God gave the government the sword, perhaps they should fulfill it. And if a Christian is to enact capital punishment, that wouldn't undermine the whole pacifistic argument. Many pacifists are ok with corporal punishment (though many are not) because God has given them direct charge over their children. So if we have a directive for capital punishment that withstands scrutiny, that is one limited case in which we may harm another. You still have a long way to go to completely undermine the other instances of pacifism.