Some may be critical of what he actually did say, but Sanders did not "support Fidel Castro."
This is actually from a right wing source so I will not link to it, but clearly it was in their interest to make Bernie look as bad as possible so they did not go soft on him:
"Sen. Bernie Sanders, the socialist from Vermont who is now challenging Hillary Clinton for the 2016 Democratic nomination, praised the Castro regime and Nicaragua's Sandinista government upon returning from a trip to South America in 1985..."
"In 1959 [...] everybody was totally convinced that Castro was the worst guy in the world and all of the Cuban people were going to rise up in rebellion against Fidel Castro," said Sanders. "They forgot that he educated their kids, gave their kids healthcare, totally transformed the society."
"So they expected this tremendous uprising in Cuba," Sanders continued, but "it never came. And if they are expecting a tremendous uprising in Nicaragua, they are very, very, very mistaken."
Sanders insisted that he did not mean to suggest "that Fidel Castro and Cuba are perfect; they certainly are not."
In 1985 Sanders was opposing the Reagan administration policies that led to the Iran/Contra scandal. Nicaragua is messed up and has been for many many decades, but the contra rebellion that Reagan was funding did not succeed in organizing any mass uprising. It sure led to many deaths though.
In a 2016 presidential debate Sanders had this to say about that 1985 interview:
What that was about was saying that the United States was wrong to try to invade Cuba, that the United States was wrong trying to support people to overthrow the Nicaraguan government, that the United States was wrong trying to overthrow in 1954 the democratically elected government of Guatemala," Sanders said.