OK. It is, to me, fundamentally wrong for the state to execute it's citizens. Simple as that.
It is wrong on a philosophical and ethical level and corrupting of both the state and it's citizens.
It is also wrong on a practical level, miscarriages of justice say, or 'mission creep' by the state among other things.
If you accept that, then you need a criminal justice system that can deal with violent criminals of this type. We have no such thing.
We need root and branch reform, a police force actually set up to deter criminal behaviour and catch real criminals, changes to the law and the court system to make conviction of violent offenders more likely and a sentencing system and prison service that can handle the worst offenders and put them away for life.
More to it than that of course, that is just the barest outline.