It seems to me that "radical ideas" are largely relative to the backdrop against which they arise, and they can be good or bad. Some abolitionists arguably had radical ideas by the standards of their time and culture, didn't they? And that kind of "radicalism" proved essential to advancing human rights and equality. I think that whether or not some of Guevara's ideas were harmful is a separate question from whether they were radical in the context of his time and location.
As for his approved means, I would agree that he was too supportive of the destructive Leninist notion that violence can perfect society. However, he was far from a historical anomaly in holding such views; historical figures ranging from some in the French Revolution and proponents of Manifest Destiny all the way to Leninists and even some Islamists have held variations of the belief that utopian violence is justifiable. This doesn't make any of them less culpable for brutality, but it puts into perspective that such ideologies aren't confined to one person but are rather a product of, among other things, human tribalism, irrationality, and desire for control.
I dislike idealization of Guevara and find him deeply flawed, but I equally oppose categorical demonization of him as if he had been some larger-than-life villain and not largely a product of his circumstances. He was radicalized after seeing the stark inequality and poverty in his region, after all, and much of that was exacerbated and prolonged by the US. I think highlighting this is crucial in any balanced discussion about him, if only because there are a lot of lessons to be learned from it about prevention of radicalization and violence by addressing root causes (e.g., poverty, inequality, and oppression) rather than just talking about the symptoms and resultant side effects (e.g., the desperate and violent measures that some radicals like Guevara and others have resorted to at many different points in history).
