Villains and Kooks

There are many people whose ideas about the future are reasonable or respected but who are misguided about the methods of achieving their visions.
Even though they may not lead to constructive results, erroneous visions deserve a special study.
Here is a modest initial selection:

Karl Marx - his main mistakes, in my opinion, were falling for the seductive idea that complex social processes may be explained and governed by a simple set of rules, similarly to those suggested by Newton for mechanical systems, and undue extrapolation of social trends of his time. Something we could learn from now...

Adolf Hitler
One of the most successful proponents of nationalism in history, Hitler gave us some very good lessons. We paid too much for these lessons not to learn from them.

Vladimir Lenin did the same thing for communist ideas.

- an author of a set of vast, vague, and unsubstantiated claims about the future, which were less unexcusable in his time than in ours.

All religious thinkers that ever lived, for mistaking emotionally charged metaphors for the objective reality?

Some people whose example you may not want to follow:

Kooks museum