I haven't read much of Marx's works, so feel free to point out anything important! Anything at all!
Communism is much larger than Marx. There were socialists/communists before and during Marx's time and there was a split after Lenin used violence to take over Russia (and I don't think we can deny Lenin's influence in associating communism with violence, and Stalin followed up with more and worse). That said, to the best of my knowledge, Marx is responsible for a grievous error that has spawned much grief.
The history of all hitherto existing society is the history of class struggles.
— Karl Marx, The Communist Manifesto
Karl Marx took the stance that the social classes are fundamentally in conflict. He was fundamentally wrong about that. Society would not be possible at all if such a statement were true.
Did I misunderstand Marx?
Marx was a student of Hegel, who had advocated that at the foundation of any idea is a conflict between two opposites. To Hegel, a constitutional government (the closest in his own conception to a free society) was the result of conflict between Masters and Servants, sublimated into a political system that would allow both to peacefully co-exist. (This is what Hegel calls a "synthesis" - the two opposite ideas are resolved into a new one that carries elements of that opposition inside).
Marx basically transplanted Hegel's ideas into the realm of economics and history (or Political Economics, if you will) because he saw politics fundamentally as being determined by economics rather than the other way around. (There is a reason for that but I'm not going into it for now.)
Marx saw history as a series of conflicts between Haves and Have-Nots in different stages, with modern industrial society the penultimate stage in that conflict. Like Hegel, Marx believed that the opposition of Workers and Capital could be resolved, but only in a new, higher stage of politics that would eliminate the economic foundation of that opposition.
That higher stage would be Communism - a society without coercive government, where people were free to do what they wanted, and economic activity revolved around free association and voluntary labor, rather than control over production.
To Marx, the foundational economic reason for the conflict between Capital and Labor was the private ownership of the means of production, depriving the working class of control over their product, their time, their place of work, and ultimately their lives. So in order to resolve the conflict between Capital and Labor, private ownership of the means of production needs to be abolished.
But that revolution of economics cannot be initiated by the capitalist class, because they are fundamentally content with that particular arrangement: They get to reap all the benefits of it, after all. So a resolution of that conflict can only come with the overthrow of capitalism at the hands of the working class, as they are the only ones motivated to resolve this conflict. (Likewise, in Hegel's philosophy, all impetus to resolve the Master-Servant conflict comes from the Servant, not the Master)
So in short: In order to achieve the actual harmony you claim already exists, people need to eliminate capital i.e. the private control over industry and production. And because capitalists would never willingly abandon their privileged position in society, the impetus for the abolition of private property
has to come from the working class.
Cue ~150 years of Marxist politics.