There's a lot of complaining about Castro here (and elsewhere) specifically because he was a communist. But if you think about it, the capitalist West has really dropped the ball on this. Most of these communist movements - in Vietnam, Cuba etc - were originally national liberation movements. And the reason why they appeared was because the respective countries were colonies, and their people were painfully (in many cases, literally so) aware of that fact.
Now, suppose you're a leader of such a movement. What's going to be your ideology, beyond just national self-determination?
Well, on one hand, you look at the guys that are currently busy denying you that, and you notice that they generally tend to be capitalist countries. If you listen to what their ideologues have to say, they notice they aren't actually saying much about your plight at all - it's all about some abstract stuff like free markets.
On the other hand, you have those communists, who constantly talk about imperialism and colonialism, and how it sucks for those on the receiving end. And you know it's true, from your own experience. And those guys haven't ever made you their colony, and aren't demanding that you become one. Basically, their talk on that subject is entirely in your favor. Well, why wouldn't you believe that they're right on all those other things, as well?
There are actually several examples of leaders that weren't initially particularly left-wing becoming more so solely because they were fighting against some Western country occupying them, other Western countries were just pretending nothing's happening (at best; at worst, they were actively helping the occupier, as in e.g. Indochina), while the Soviets were ready and willing to supply food, arms, and everything else you need to fight. Of course, it came with ideological strings attached, but beggars can't be choosers.
Castro, for example, was not a communist when he first started to participate in violent resistance. He was anti-American, and specifically anti-American involvement in the countries in the region, which then consisted of backing dictators like Batista and Trujillo. It was sometime after he started down that road that he became to radicalize along Marxist lines, especially after several bitter setbacks (that also made it clear that fighting against US requires a powerful ally to succeed).