Marxism in practice requires riling up the lower classes and whipping them up into an emotional frenzy in order to bring about a revolution. This always results in violence and literal class warfare. Marx himself spent his life trying to do this and advocated for it.
The Q relies on an accepted premise around power structures. Change, whether gradual or quick, can be effective or ineffective. It depends on the source of the change and whether society accepts the changes e.g by leading those changes, rather than those having them impose.
Part of the constraints of capital punishments in modern society is that they are quick and humane. We don't torture people. Many forms of experimentation on criminals would likely be equivalent to torture and are inherently unethical because of the recurring harm they may cause.