The death penalty is wrong. Murder is wrong, even if you're murdering a bad person.
The last power a state should have is the power to murder its own citizens, even if they're "bad people."
Whenever the people gift the state a power it must always be considered "what is the worst possible way this power can be abused or expanded?"
Our government can now murder US citizens without trial, a power it would be harder to justify if they weren't already given the power to murder "bad citizens."
FOR EXAMPLE:
There was recently a case against the federal government brought by a journalist who is an American citizen. He reports on matters in the middle east. During his time in Syria there were 5 drone strikes extremely close to him. He began to get the suspicion that he may have been erroneously put on the kill list (yes. we are a civilized society that actually has one of those. Its official name is the 'Disposition Matrix')
He sued trying to find out how he got on the kill list. The government made the case that he doesn't have the standing to bring the case, because he doesn't have any hard evidence he is on the kill list.
The government also said that the kill list is privileged under the "States Secrets" clause and that even if he were on the list, the government had no obligation to let him know that he was on the list or how he ended up on it.
https://www.rollingstone.com/politics/politics-features/how-to-survive-americas-kill-list-699334/
In short, we've delegated the power to the government to kill "bad citizens." Now they have granted themselves the power to kill "bad citizens," with drone strikes, without a trial, nor do the citizens have the ability to remedy a situation in which they are targeted for state-sanctioned murder.
The states duty above all is to protect its citizenry. Giving it the power to kill its citizenry is immoral, impractical and counterintuitive to the duty of the state.
Don't confer powers to the state that when used the worst way possible can do massive harm. Whenever the state is given a power, it is only a matter of time before that power is abused.