Why might executing a convicted murderer for the sake of deterrence be an example of treating a person as a mere means?

ESSAY PROMPT
According to deontologists, the morality of an action (its rightness or wrongness) depends upon your reasons for performing it. Underlying motives determine an action’s moral character. Some deontologists argue, on this basis, that it is wrong to execute convicted murderers for the sake of deterring other would-be murderers. Do your best to reconstruct such an argument by answering (a)-(d) below.

(a) What does it mean to treat a person as a mere means?
(b) Why is it wrong to treat people this way?
(c) Why might executing a convicted murderer for the sake of deterrence be an example of treating a person as a mere means?
(d) Evaluate the argument in (c). Is it true that executions for the sake of deterrence are immoral?