You wouldn't to get in a fistfight with Fernand Mondego. He's a big dude, and a passionate one. As a young man he longs for Mercédès; his passion for her is so great that he vows to kill Edmond rather than give her up. That said, he's pretty much all talk. He can't bring himself to kill Edmond, not after Mercédès threatens to kill herself if he does, and so he does the next best thing: he whines and he nags and he nags and he whines. Really, Fernand is a whiner and a mope above all else. He doesn't even relish the opportunity to put his rival away. What kind of villain doesn't enjoy sticking it to his nemesis?
Even after Edmond is gone, he doesn't change his tactics. He just waits and mopes and hopes Mercédès will consent to some sort of pity marriage. Only his conscription into the army breaks his cycle of whining and depression and, well, the next time we seem him – as the Count de Morcerf – it's a whole 'nother story.
Oddly enough, Fernand doesn't really get much screen time – or, uh, page-time as it were – in the second half of The Count of Monte Cristo. By the time the Count has tracked him down, Fernand has already secured himself a title, settled down with Mercédès, and had a kid. We learn the most important details about Fernand's post-Marseille life through Haydée, who tells the story of her father's – that is, the Pasha of Yanina's – betrayal. Fernand, tasked with protecting the Pasha from the Turks, actually sold him out.
That's a good way of thinking about Fernand, actually. He's a sell-out – not in the punk-rock-authenticity way, but in the sense that he doesn't know the first thing about loyalty. Though he's Spanish by birth, he has no problem with fighting against his own country and, well, he doesn't mind letting people die in exchange for a lot of money; he even sells Haydée to a slave dealer to make some cash.
So, when the Count finally shames Fernand and reveals himself, we don't feel so bad about Fernand killing himself. After all, back then suicide was a socially acceptable response to being dishonored. Does this mean we should be cool with it? Not necessarily. By the end of the book – a few suicides later – the Count has misgivings about his whole ten-year foray into revenge, despite the fact that he doesn't technically pull the trigger. It's really pretty difficult to sympathize with a man who betrays people for money and sells innocent women into slavery but, you can understand how the Count might have qualms about what happens, especially considering that it leads to the widowing of Mercédès.