This kind of death is undignified. In one sense, all death is undignified. In the final analysis, the question is not the dignity of death, but its equity or justice. There's no opportunity of to reform the criminal. Justice is the goal of punishment, not reform.
Capital punishment violates human dignity.
It is specifically because of man's value and dignity that we punish his moral wrongdoing. We hold men morally responsible because of dignity. Arguably it is undignified to force rehabilitation on free moral agents who don't want it. The Catholic position against capital punishment is somewhat ironic given their position on purgatory, in which even when God forgives a sinner, still he must suffer for his own sins. Show how capital punishment was assumed in the New Testament.