Warning: Spoilers for both Death Bringer and Kingdom of the Wicked.
Firstly, each character's traits.
Doran is one of those characters everyone despises. He is a stupid, mindless thug. But he's strong. Kitana gets a little more respect (ruthlessness commands respect). She's a crazy, spoilt brat who reminds me so much of Opal Koboi from Artemis Fowl. Sean's a prat and I'm sorry, but Elsie is a loser. She could've been a heroine, but she wasn't. And that is a credit to Derek Landy's writing, because she's a realistic person. At least she walked away and didn't directly take part in the mad rampage.
Apart from Elsie, they're all freaking psychopaths.
Vile is a warrior fueled by pure, focused hate. He's ruthless, driven and hard to injure, but once he's properly hurt, he has no magic tricks to save him. However, he is an extremely dangerous Necromancer. Darquesse is in possession of her true name, insanely powerful and capable of performing what seems to be raw magic. She's adaptable and her power is constantly growing, but she only comes out when Valkyrie's life is in mortal danger. Also, she's relatively inexperienced when against Vile, but knows far more than Kitana.
Experience and training are really the only things that enable Vile and Darquesse to win against Kitana, Sean and Doran. Without that, counting on raw magic, it's one (Darquesse) against three (Kitana, Sean and Doran). Talented, efficient and deadly as Vile is, he doesn't possess the same calibre of magic as they do. But in the long term, he's better. Because the three teenagers only have their extreme powers because of the sorcerer Argeddion, who knows his true name. Darquesse and Argeddion are really the only two with the power of their own names in the fight, but Argeddion has lent his magic to his three protégées.
And this raw magic is lethal.
Kitana, Sean and Doran, normally ordinary mortals, are able to skip the years of training done by everyone else to even approach their level. They work on pure instinct, like when Skulduggery tries to shoot them and they throw up a forcefield, not having known themselves that they could do that until it happened. Because of this strange new power they become cocky and self-serving, with Doran assuming a tough-guy act just because he can.
Of course, the protagonists still have some advantages, Before they become their evil counterparts, Skulduggery and Valkyrie are able to work together, instead of battling for dominance. Oh, platonic Valduggery, this ship develops so much more in KOTW. It’s inevitable that the tensions in the group of psychopaths tear them apart eventually.
Darquesse is a very high-functioning sociopath, so even though she takes glee in killing, she’s still masterfully good at what she does and doesn’t make stupid mistakes like Kitana – taking unnecessary risks as she becomes even less wary. Even when Darquesse is taken by surprise by their impossible recovery, Vile is there to step in – like a boss. I’m not going to go into the manoeuvres of Argeddion. Suffice to say that Val works around them eventually, even though they are very clever.
In the end, it all boiled down to this: Val and SP are trained, so much more than Kitana and co. would ever consent to being. That enables Darquesse to pull the recovery-from-decapitation stunt, ,and Vile to work around the impossible magic they have. And that’s the thing. It really is impossible magic. So impermanent. It couldn’t last, and our protagonists capitalized on that. It was a terrifyingly close fight, but they won. And unless Kitana, Sean and Doran wised up and learned that power comes at a price, that they couldn’t expect to be so good at something without working at it, they could win again.