The practice of capital punishment has a long and intense debate over it that has lasted for centuries. There are powerful arguments both for it and against it. Nineteen states do not allow it, and thirty-one states do, as well as the federal government and military. Because of the finality of this procedure, that intense debate will continue forever.
Christians of good will can make a good case for either retaining or outlawing the procedure, and neither is right or wrong. It is a judgment call as to whether you think capital punishment is appropriate for heinous crimes like mass murder, as well as a necessary deterrent, or whether you think any kind of judicial execution is murder, especially because it has been proven that juries are sometimes wrong.
You cannot say that capital punishment is against God’s will. St. Paul wrote on how God entrusts governments with keeping order in a chaotic and violent world. In his view sometimes violence is needed to suppress and restrain violence: “The one in authority is God’s servant for your good. But if you do wrong, be afraid, for rulers do not bear the sword for no reason. They are God’s servants, agents of wrath to bring punishment on the wrongdoer” (Romans 13:4). Here are two things of which you can be sure: the debate over punishment appropriate for the worst of crimes will continue indefinitely. So will extremely violent crime.
Only in heaven will that argument cease.