Why was Karl Marx so critical of capitalism?

Because it causes a lot of misery for a lot of people, just to make a small minority insanely rich

He saw that workers were largely being deprived of the fruits of their labor. He wanted a classless egalitarian society where workers worked co-operatively.

Because when he was writing workers had no protection against exploitive owners.  

He was well intentioned but misguided about how to correct it along with ignoring human nature.  He envisioned an egalitarian society with plenty for all and a withering away of the state.   His ideas on achieving that were worse than worthless.  He did not have evil intentions, just really bad ideas that worked opposite of what he thought he would accomplish.

To much diversity, individual freedom + liberty, too prosperous for all, not enough oppression/strong control

Because he knew it could only lead to an escalation of income inequality and loss of power for the lower classes 

He was jealous of people who were successful because of it.