Specific political ideologies that use Marxism as a philosophical basis for their platforms, like Marxism-Leninism, Stalinism and Maoism, are responsible for mass murder. And yes, I would consider it okay to privately discriminate against people who publicly espouse such ideology.
So it's sheer coincidence that it has led to starvation, slavery, and mass murder every time it's been implemented?
100 million dead bodies is enough to convince me that it's a bad idea.
I'm afraid that the burden of proof is on you at this point.
No. It's the consequence of the fact that all that have been implemented are offshoots of one particular political ideology, namely Marxism-Leninism. And that is the case because it was the only Marxist-inspired ideology that was brutal enough to prevail against attempts to suppress it - more liberal-minded Marxists that put more emphasis on democracy, such as the German Spartacus League, were violently suppressed.
After that initial period, practically every left-wing movement was quickly co-opted by Soviet Russia / USSR, if not created by them outright, and so it inherited the ideology. In some cases, viable alternatives were also violently suppressed by Soviet agents (as was the case with e.g. anarchists in Spain during the Civil War).
Stalinism, Maoism etc are all derivatives of that same system.
Socialism and Marxism are not synonymous, no matter how much you would like to pretend that's the case.
Edit: in my original post, I said that "self-described Marxist regimes murdered 100 million people".
Which of the social democracies you mentioned explicitly describes itself as Marxist?