Every socialist revolution back to the Paris commune has been met with violence from the capitalist powers of the world. You can visit wikipedia and count how many countries participated in the Russian civil war on the side of the whites. Try to count how many times the United States tried to assassinate Castro while you're at it.
Since all socialist revolutions are met with the full ferocity of the capitalist states, it's not clear how they can win militarily without becoming a dictatorship. A military is perhaps the social organization least amenable to democracy and most amenable to dictatorship. It's no surprise that socialist revolutions which succeed via military dictatorship and never demobilize (because they can't, because they are constantly under threats real and perceived) begin to reflect military dictatorship in their social structure.
No socialist revolution has succeeded in an industrialized nation. They have all taken place in the periphery, either in colonies of imperial powers or extremely weak states like tsarist Russia. None of those successful revolutions managed to industrialize without devolving into centralized, dictatorial rule. Socialist thinkers back to Marx thought that socialism is enabled by industrialization; not something that can be implemented beforehand. This sounds entirely consistent with history. Note that the State had a crucial role to play in the development of capitalism. You can't pretend that capitalism isn't born of and sustained by violence when it has been instrumental in every development that led to capitalism's ascent, from forcible enclosure of the commons, to imperialist plundering of non-industrialized nations, to strikes broken by national militaries, and so forth.
If it doesn't seem so violent now, maybe it's because resistance to it has been thoroughly purged from your milieu and the violence that remains has been pushed slightly out of your perspective. Nothing to see. There's no possible way capitalist nations were responsible for, say, the world's largest ever cholera outbreak in Yemen.
Socialism is the democratic control of the means of production. All attempted socialist revolutions devolved into a system of bureaucratic control by a centralized government. It sounds like socialism hasn't ever been achieved.
> One can think it should teach a lesson... but some people just want to believe.
Most socialists think that an alternative system is necessary because capitalism with all its contradictions are untenable. Treating socialists as shallow thinkers just makes you look shallow. But it is quite a fashionable look these days.