I'm hoping that one of the Cuba experts would drop by to tackle this one, but since this question has been up for a while, you may be interested in checking out some earlier threads asking about assessments / controversies surrounding Guevara and Castro. There have been loads of threads, but hopefully I've pulled the ones that are most relevant to the question of why popular opinion varies so widely.

###Ernesto "Che" Guevara

• Was it the truth behind the critical controversy surrounding Che Guevara? Was Che a murderer, a homophobe, and racist who needs to be viewed much more critically? - featuring /u/ainrialai

• Che Guevara is a hero to some and a villain to others. What factors have influenced such widely differing perspectives of Che Guevara? - featuring /u/WuTangGraham

• Jon Lee Anderson, author of Che Guevara: A Revolutionary Life, says in a Q&A: " I have yet to find a single credible source pointing to a case where Che executed 'an innocent'." Can anyone confirm or debunk this? And how accurate are the other answers he gives? - a long discussion thread

• Why is Che Guevara somewhat of a pop icon even though he is a Communist Revolutionary that was Castro's right hand man? - featuring /u/ainrialai

• Why is Che Guevara such a pop culture icon? - featuring /u/BlueStraggler

• How the story is of Che Guevara is painted in U.S. schools? In South America he is like a hero. - featuring /u/ainrialai

###Fidel Castro

• How was Castro as a ruler? - featuring /u/Jirodreaminsushi; see also the link by /u/anrialai

• Why, throughout history, have militarily focused communist thinkers like Che Guevara or Fidel Castro been hailed and popularized more than those democratically elected like Salvador Allende?

• Was Fidel Castro a "good guy" for lack of a better term?

• Why is Fidel Castro so often seen as such a villain and Che Guevara seen as such a hero when they worked side by side on the same campaign?

• Why do Cuban Americans hate Castro so much?

All of these posts have been long archived, so if you have follow-up questions for any of the users, just ask here and include their username to notify them

It is hard to really answer your question, because it depends a lot on politics and whether you agree with their actions. These are fundamentally ideological figures who took their actions for the sake of their ideology, and your appraisal of them will vary significantly depending on whether you agree with them, especially outside of an academic context. That's why there's so much diversity of opinion here, because they're fundamentally ideological figures, and unfortunately there's not too much more I can say without diverting from the function of this sub.