Or is this a world where only nice people get to live forever?

Neither Hitler nor McCarthy were still 'dictating' when they died. Lack of death doesn't mean that everything would remain the same forever.

Stalin's death lead to a good change.

When somebody stay in power for too long it leads to stagnation.

Human civilization has other measures against such stagnation (such as 2 terms limit for presidential position).

However limited life span still adds extra protection against society stagnation.

It's not obvious if benefits of unlimited lifespan outweight potential problems.

I think that doubling life span would probably be beneficial overall, but it's hard to say if further life span increase would.

No, I'm talking about the elimination of aging and natural death. People would still be able to kill themselves and others.

The legions of true believers would be able to dictate terms to enlightened society with the mere threat of violence.

I really don't think you've thought this through.

Think about what you're saying here. You think no one in Germany was rational? They would have had the same risk of dying we would. Hitler was only powerful because there were people willing to follow his orders.

And there was no shortage of true believers on the Allied side either (think patriotism), in any case.

You would have to be pretty damn patriotic to want to give your life to preserve a nation-state you expect to outlive anyway. If I'm going to die anyway, it doesn't make much difference; it makes sense to risk my life for some things. If my life is eternal, what principle could I possibly, rationally put ahead of my survival?

Eternal life gives the insane an advantage over the sane, and the idea that the insane are as likely to be good as evil seems like a risky bet to me.