Because the optimum is a public process which proves their guilt beyond reasonable doubt so that every good person supports their punishment and has the confidence (certainty of guilt) to support it publicly.

But if the choice is between no punishment and somebody gunning them down in the street or droning them, i prefer the latter.

Court processes are useful when guilt is uncertain at first look and you want to increase certainty. But dictators and their close supporters, the certainty is often sufficient by nature of many their actions being public. Sometimes they literally go on TV and declare they're going to a foreign country to kill their people and take their land. At that point, it only becomes a matter of making sure you have the right person.

And don't forget the victims. Many authoritarian regimes don't kill opposition outright (for various reasons) but imprison them instead. Such a victim knows many of the people (cops, judges, informants, etc.) responsible for / guilty of falsely imprisoning them. After a regime change, the victims go free and have often more knowledge of the offenses than can be proven to a court by the simply virtue of being there and therefore have more than enough confidence to deliver a just punishment.