At its core, this play is about the sin of being a selfish miser. Hence the title, The Miser. This English title is translated from Molière's French title for the play, L'avare. But the words translate pretty directly: in fact, the SAT word "avarice" means greed. Avarice, avare: potato, potahtoe.
The term "miser" refers to a selfish person who obsesses over hoarding as much money as possible. No amount will ever be large enough for him to relax. And at the end of the day, Molière's play shows us miserly Harpagon in order to warn the audience away from ever becoming like him.
Unless, of course, you want to be hated by everyone around you. If you want hatred double-quick, hop on the miser train.