In a perfect world, I’d agree - if law officials were to perfectly convict guilty people each time, and if execution methods were absolutely flawless with painless/instant death. However, that’s not what happens in the real world.
There’s an amazing video by Jacob Geller on execution methods, where he talks about the history and the present of death sentences, but in short, not an insignificant amount of people get falsely convicted and then executed, and the execution methods US are using focus more on “appearing humane” rather than effective/painless, and the incompetence of the executors turning some of those executions into hours of agonizing pain.