Karl Marx's ideas were evil because they were based on hatred and antagonism (class struggle) rather than tolerance and diversity. In his proletarian utopia, only one kind of human could survive. Since that human did not exist in the real world, when the system was put in practice, existing humans had to be transformed into that type or be killed off. Luckily nobody was able to implement pure Communism as Marx had dreamed, but many were seduced by the dogma and tried it out on real people, causing untold amounts of suffering.
Let's look at the system when put in practice:
1) Political Failings:
Communism tried to solve existing social problems and inequities by concentrating power in the hands of a small elite unaccountable to anybody. Without a system of checks and balances, this system inevitably lead to large excesses by these ruling elites - more often than not leading to genocide and mass murder.
2) Economic Failings:
Communism concentrated economic decision-making into the hands of a small elite, unaccountable to anybody. Time and time again, this way of running an economy proved much less efficient than a free market for goods and services.
Think of an analogy: if google maps were updated by a centralized institution, would it be as efficient as the model it adopts now (open source, with millions of users updating the data in real time)?
With such an inefficient economy, most communist societies ended up in dire poverty; for the most case, the regimes which survived had to radically transition from the communist economic model to a freer market.
3) Foreign Policy Failings:
Communism was an aggressive dogma, and communist societies were required to spread the ideology by force around the world. That is why you had Cuban fighters in Angola or Russian soldiers in Afghanistan. Often the ideological need for conquest coincided with more practical needs - with such a poor economic model, the stronger communist countries (such as the USSR or China) needed resources from other countries in order to survive economically.
I never understood why people often say "it's a nice utopia, it just can't be put into practice". Communism has a bad reputation in the civilized world for very good reasons.