There is a problem with Roger Scruton's reasoning. It is not a moral problem, but a syllogistical one.
He states that:
1) Respecting the dignity of a person means he should be treated as a free and responsible individual person. (I agree)
2) Therefore, if an individual person ought to be free and responsible, he therefore ought to be faced with the beforehand known inevitable legal consequences for his actions. (I agree)
3) Therefore, not giving a murderer (or any criminal) the punishment he deserves, is a violation of his dignity, because he is not treated as a responsible individual. (I agree partially; but let's follow this reasoning)
4) Therefore, a person who committed the crime of murder, he should be punished by death; except, if the relatives of the victim sue for the murderer's mercy. (I do not agree)
So, there is a missing link in sir Roger's chain of logical arguments, between step (3) and (4). Scruton suddenly jumps from society's duty of establishing legal certainty, to a discrete personal opinion on the measure of severity of penalty in the case of certain crimes. So this is a personal opinion of him, though eloquently formulated, without correct argumentation.