who was in the wrong here? was mccarthyism justified?

There were communists as high as the presidential cabinet. He was completely justified. I don't have the numbers onhand because I'm traveling, but modern investigations proved that he was correct a vast majority of the time with his accusations.

Schools in the US teach him as an extrajudicial monster only looking for power.

Arresting people for being communist violates their constitutional right to free speech. the amount of commuists he actually found or not, and whever he was just doing it because he wanted power or not is irrelevent. It's simply unconstituional.