This week Jews around the world commemorate the Maccabean revolt in the celebration of Hanukkah. But what actually happened? Was Antiochus IV as tyrannical as portrayed? How was it that the Judeans under the Macabees were able to defeat the Seleucid Empire?

I think the problem with providing a comprehensive answer is that I can only think of two sources to draw from, both of arguable historical merit (for this particular question):

The book of the maccabees (1 maccabees), which is supposedly contemporaneous (written about the second have of the 2nd century BC), but probably provides a pretty one-sided narrative, without any other work to back it up.
The first book in The Jewish War by Josephus Flavius. Written some 200 years after the revolt, it starts by depicting the Maccabee revolt as part of a larger conflict between the Seleucid Empire and Ptolemaic Egypt. In that later retelling, the original massacre of Jews in Jerusalem is done after factions backing Ptolemy VI attempted to take control of the city.

I'm aware that these sources are in possible violation of the sub rules (please delete my comments if they are), but you could try reading those until someone comes up with a more qualified answer (I didn't expand on the narrative of each source because I wasn't sure it would be acceptable).

As for your second question, I think it's easier to understand the reasons for the success of the revolt when seen in a larger context. The Seleucid empire was in a state of disintegration after being defeated in the Roman Syrian war. Losing most of their holdings in Asia minor, they were left in a weakened state which was exploited by both Egypt and the Parthians (Antiochus IV himself died in battle with the parthians in 164 BC, 4 years before the end of the Maccabee revolt). Once again, others should elaborate, but you can read more about the weakened political state of the Seleucids (at least from a Roman perspective) in Polybius's The Histories (29.25-29.26), for example.