It's worse than murder because
- the state MUST be held to the highest standard;
- the state has the resources to do other than kill;
- execution ends up costing the state more than life imprisonment or better solutions;
- because someone who is ready to murder typically already does not fear punishment of any severity;
- because resources spent on decreasing economic inequality decrease the murder rate far more effectively than capital punishment ever can;
- etc.
We / our society need many more like that guy, but I wouldn't wish that level of stress on anyone. I'll bet he gave two years off his eventual lifespan for every year undercover. On the other hand, lifespan would be worth so little in a white-supremacist christofascism.