This is just plain misleading. Marxism/Leninism is not the only school of thought against capitalism, or that distinguishes between proletariat and capitalist class in the capitalist systems.
And I would argue that
1. the "slavesystems" you're talking about (I guess you refer to soviet russia, china and other similar dictatorships) never applyed the Marx theory, but they rather implemented a state capitalism system (IMHO because the idea of dictatorship of proletariat as a means to reach communism is plain stupid).
2. even considering those cases, which I agree were fundamentally flawed and unjust, there's still debate about whether they caused more or less death and suffering than capitalism. I think this answer [0] on Quora gives a good summary of the main points in that regard.
[0] https://www.quora.com/What-has-killed-more-people-communism-...