Why must all evil people of the world be one ideology to a lot of people?

Ideally, yes. But this runs counter to the very nature and means that movements exist for.

A movement must be to some degree intolerant of others-this is precisely because a movement offers a sense of identity to those within it. The only way to fully accept the identity of a movement is to assume it is better than others (why else join it, after all?). However, this naturally regresses in time to a desire to expel (or more likely, deny) those bad qualities in our group.

However, a quality-good or bad-is meaningless if not attached to someone. No doubt, Hitler had more than his fair share of bad qualities, but his role of a scapegoat is second only to the sheer magnitude of the horrors he unleashed. In order to be "rid" of the bad qualities, we seek them in others. Psychologists call this "Projection". A shortcut to projection is to claim that someone like Hitler possessed all the qualities we ourselves have but are too afraid to admit.

Now since we identify with a group and sacrifice some of our individuality as a result, the group itself requires us to retroactively project flaws onto people like Hitler.

So, to summarize, a group retroactively projects onto men like Hitler to force their rivals onto the defensive and to smear them in the process.

But all movements, regardless of the doctrines they preach, the ideas they promote and the ends they seek to achieve, generate hatred, intolerance, stubbornness and a will to obey some sort of authority-be it a person or even something like a Constitution.

This is an inescapable reality of movements, and while some are certainly worse in both means and ends, all have moments of inhumanity, irrationality and general jackassery.

By definition all dictators are governmental control types. They can't be fully conservative as a result. They can be liberals though. When a left wing person points out a "conservative dictator", they mean a religious person (usually they just claimed the religion to gain the power in the first place) who used government to subjugate people. There have been numerous fully left wing dictators and they have been the worst. Hitler, Stalin and Mao were all left wing. They killed 100 million people.

It is important to identify the underlying assumptions of a given philosophy that manifests in various forms of evil. Simply just saying it's evil because it had bad results is worse than pointless.

Demagoguery is a useful tool to control the populace, no matter what your ideology or the ideology of your opponent.

Indeed though some ideologies do call for certain set of rules that can apply to deaths.

We probably will never progress. If we do it will be very little very slowly.

Well said.