Bastard used to be a not nice thing you called a child whose parents weren't married. But now it's a more general insult hurled toward a jerk or bad person. Bastard can also simply mean "fraudulent."
This is a great example of how words change in meaning over time. Today, if you were called a bastard, it probably has nothing to do with whether or not your parents are married: it just means someone doesn't like you (unless they call you a magnificent bastard, which is a compliment). Bastard can also mean phony or fake, like a bastard version of French that is not correct. When you think bastard, think illegitimate.