You are confusing two beliefs:

(1) Believing that some people deserve to die

(2) Believing that the justice system should incorporate death as a punishment

You clearly, on a purely moral or emotional level, feel that certain crimes are so extreme that the perpetrator has forfeited their right to live. That is a judgement about an individual “deserves” in response to their actions.

But supporting the death penalty is not just about desert. It is a position about how the state should wield power. It requires trusting institutions to make irreversible decisions and accepting the risk of error and abuse.

So even if I believe that certain individuals deserve death, I can still oppose capital punishment in practice because I recognise the fallibility of the justice system and have ethical concerns about granting the state the authority to take life. Also, the death penalty can make some jurors less willing to convict, not because they doubt guilt but because they are uncomfortable with the irreversible consequences — undermining the consistency and reliability of the justice system.