Danglars's motive is pretty darn boring: he's jealous of a younger, more attractive, more talented co-worker. He's sort of like Dwight on The Office, minus the glasses and nerdiness. Boring or not, his motive leads to a despicable action and a terrible outcome. Remember, too, that he convinces the mopey, aimless Fernand to channel his directionless violence into a non-violent but still damaging action. He intimidates Caderousse, too, convinces him not to speak to the authorities – not that Caderousse needs much convincing. Danglars is, in short, the ringleader. His is the hand that writes the denunciation and his is the scheme that sets the 24-year-long saga of Monte Cristo in motion.
He knows pretty early on that his actions could have some serious repercussions if Edmond were ever to bust out of the clink; he splits for Spain before he can ever become captain of the Pharaon. Ultimately, his craftiness can't quell his fear of Edmond, which, of course, says a lot about Edmond himself.
Of all the people who did Edmond Dantès wrong, Danglars is the last one you'd expect to get off easy (relatively speaking). As ringleader of the plot against Edmond, he set the revenge ball rolling, and yet he escapes relatively unharmed. Sure, he loses his family, his reputation, and his money, but he doesn't really care about the first two anyway. Also, he isn't driven mad, and he doesn't kill himself.
Why would Dumas choose to let Danglars off the hook? Well, precisely because he's the last one you'd expect to get off the hook. Having made his fortune by speculating on stocks – that is, by chance, with a little bit of cheating and corporate skullduggery thrown in for effect – it just seems appropriate for Danglars to be struck down by the hand of Fate as embodied in the Count of Monte Cristo. Throw in the fact that he's mean and he's crude and he has bad taste in art, and you have a classic villain, unworthy of pity.
Are you really getting angry now, angry that that dirty son of a gun Danglars got off so easy? Good. Now you know how the Count feels. Back before Edmond has his moment of clarity and realizes he might be taking things a bit too far, you can bet your bottom dollar he was going to let Danglars rot in Luigi Vampa's hideout for a lot longer. But, as he watches the insane Villefort tear his hair out digging for his "dead" son, the Count has a change of heart. "Enough!" he says. "Let that be enough, and we will save the last one" (112.101). For most of the book we marvel at how patient the Count is, at how far he's willing to go in order to avenge himself, but the moment he pardons Danglars is as amazing as any other. To have persevered, to have gotten to the point where he can exact judgment is incredible, but to give up that chance when the time comes is truly miraculous, and a reflection of the Count's ultimate strength and kindness.