I think you're badly misreading a correlation as causation here.
There is no implication that a slide towards socialism causes hideous evil dictatorships, it seems far more to me that hideous evil dictatorships like to use the pretext of socialism to have the state claim control of the national economy.
If you look at almost all of these nations that you cite you will realise that they have been taken over by a revolution and made a massive leap into socialism, not the gradual slide that you imply is happening around the world.
This also invalidates your claim that the government system prevents things like Hitler from happening, it doesn't. Germany was quite far right along that line when Hitler came to power (and in actual fact Hitler never moved it left along the line - I completely disagree that Nazi Germany was socialist), Russia was quite far right along the line when the communists came to power.
