Execution does not serve justice
Of course, a dead person can not commit any future crimes. However, the purpose of criminal punishment is not only the prevention of new crimes both by convicts and other persons. Criminal liability is aimed at correcting the offender, and "aims to contribute to the restoration of social justice. The conviction of the culprit is the basis for the recovery of property damage and financial compensation for moral damages from him.” As we can see, the moral harm to relatives of the innocent victims can be compensated financially, and not by shooting the culprit.
Criminal liability, as well as justice in general, should not be an act of retaliation or intimidation, it is impossible for a killed man to understand his actions and repent. Here's a quote from Andrei Zhuk's letter: “Expressing his standpoint, the Minister of Justice says that while considering this issue we can ask about the opinion of the victims' relatives. Emotions and vengeance are not justice. In this case, there is no need for any courts: they should just give the relatives weapons and let them make a lynching...".
The death penalty completely negates the principle of rehabilitation in society and respect for human rights. Treating criminals as undesirable elements, which needs to be eradicated in a society, is a cruel and totalitarian approach to crime prevention, unworthy of a civilized society. Criminals are not non-humans, but are humans who have been brought up by the society itself. Crimes are often committed during extraordinary moments of life, with particular stress and exceptional circumstances or as a result of many years of suffering from abuse and violence, inaction of law enforcement and social agencies.
The experience of many countries that have abolished the death penalty indicates that dangerous offenders can be safely isolated from society without resorting to execution. Persons who are sentenced to death make up only a small proportion of all offenders. Executions are mainly aimed at depriving a man of his life and preventing him from committing crimes in the future, regardless of the fact that the overwhelming majority of such people wouldn't commit such crime again.