One could mention a car. When we allow to produce cars we are fully aware that many people will be killed in road accidents. However we don't think about banning cars.
BECAUSE CARS ARE VERY USEFUL METHODS OF TRANSPORTATION!
The death penalty is not useful in anyway.
No person MUST commit awful crimes.
that's why we try and imprison them when they do, there's a benefit in imprisonment, (though we could argue about how effective the system is,)
but why do we need to kill someone for their crime?
A government can't get caught up in killing its own citizens, no matter what they'e done, and it certainly can't risk killing an innocent civilian it is supposed to protect.
Your analogy with the pianos and the cars is just wrong-headed. Killing timothy McVeigh served no purpose, but you could argue that driving to work served a purpose even if you ended killing someone by accident, you could argue that moving a piano served a purpose, even if that, too, killed someone by accident.
But driving drunk serves no purpose, so we outlaw it because it can lead to accidents, the death penalty serves no purpose,Killing Timothy McVeigh had no purpose, no positive result. So executions should be outlawed too because there could be accidents.