There is a lot going on in this painting. First of all, if you don't know what the word "miser" means, go look it up; it's a great word. The miser lies on his deathbed here, torn between good and evil. Death, the skeleton entering in through the door, ready to strike with his sharp arrow, indicates that his time is almost up. The angel next to him urges him to place his trust in Christ, pointing to the crucifix in the window. The miser is about to look at the crucifix but is also reaching for the money being offered him by a demon. Indeed, there are a lot of little demons running around in this picture, signifying that this man is well on his way to joining the rest of the demons in Hell because of his wicked lifestyle. We see some of his wicked lifestyle played out for us. At the foot of the miser's bed is a scene from his past. We see the miser himself, dressed in green (the color of greed), holding onto a crucifix with one hand and, with the other, reaching for a bag of money (again being handed to him by a demon, implying that he is getting the money through immoral means). One of the demons is even holding up a papal indulgence (payment made to the church in order to acquire salvation). Ah! the greed and corruption of many of those who claim to be so humbly devout in their religion! Bosch was onto something here; this was painted before the Reformation, when Luther attacked the idea of papal indulgences.
We see demons, a greedy miser, and Death himself in this painting, but the presence of the angel is meant to show that no matter how evil a man has been during his life, he can be saved if he asks for forgiveness before dying. But which do you think the miser will choose? His room already communicates the air of the Lake of Fire with its color scheme—a fiery orange bed and a red ceiling. Bleak? Actually Bosch hints at satirical humor. The whole painting has the hint of a comic edge to it with all those cartoonish-looking demons—it's making fun of those greedy religious leaders (here, namely the Catholics) who are really nothing but money-loving swindlers who will come to their own demise because they can't refuse a bag of money even when it's offered by the devil.
Bosch's most famous triptych, The Creation, consists of a left panel for Earthly Paradise (the Garden of Eden), a right panel for Hell, and a very curious center panel for what is called the Garden of Earthly Delights, wherein a lot of really strange things are going on. That's a really weird one; I won't post it here.