Who is considered good in this play and who is bad?

Don Juan is undoubtedly a bad character. A reckless, selfish libertine, he treats women appallingly, leaving a string of broken hearts in his wake. In addition to being a serial womanizer, he's also an atheist—a very serious offense at the time—as well as a coward and a hypocrite. Worse than all of this, Don Juan is completely unrepentant, which is why he's consigned to Hell for his many sins.
Sganaralle, Don Juan's servant, is not much better. He's a devious, greedy coward with a smattering of shallow learning, but does at least possess the saving grace of a ready wit. He also partially redeems himself by his attempts to warn men of the threat that his master poses to their womenfolk.
Elvire is notable for being the only genuinely noble woman to be seduced by Don Juan. Abducted from a convent by our anti-hero, she is then subsequently abandoned, as the importunate rake moves on to his next conquest. As we might expect, Elvire is initially furious at being discarded like a worthless piece of garbage, and her broken heart cries out for vengeance. But all that time spent in the convent wasn't wasted, and Elvire somehow finds the strength and wisdom to convert her implacable wrath into the purest piety.
Elvire establishes herself as an intermediary between Don Juan and the Almighty, earnestly striving to make the incorrigible playboy change his ways. Unlike Don Juan's phony conversion, Elvire's religious convictions are totally sincere. Her inherent goodness arms her to resist Don Juan's second attempt at seduction, something he's never tried on the same woman before. The good, in the shape of Elvire, triumphs over the bad, in the shape of Don Juan, just as what was regarded in Molière's day as the true religion—Roman Catholicism—triumphs over the twin evils of atheism and superficial faith.