Valentine's evil genocidal plot marches forward in City of Ashes as Valentine steals Maellartach, the soul-sword. It's surprisingly easy to take an artifact of such power. Valentine merely walks into the Bone City as though it's Walgreens and just takes it.
What does he need a sword for? Well, if he can perform a complicated blood ritual, he can turn the good sword into an evil sword, which will allow him to summon demons that he can control with the Mortal Cup, which he stole last book.
Valentine is still bent out of shape over this whole racial cleansing thing. Like Voldemort and his hatred for mudbloods, Valentine hates Downworlders. Only the pure will survive his reign. This isn't a popular opinion with the Clave, which is supposed to defend the Downworlders, but Valentine justifies his plan, saying,
"It takes a true patriot to dissent, to say he loves his country more than he cares for his own place in the social order. I've been vilified for my choice." (2.13.86)
If Valentine is a true patriot, then so is John Wilkes Booth.
We're often told that Valentine is charming, but we just don't see it. Once the charm wears off, Clary notices, "Without the surface patina of charm, he seemed—empty" (3.19.124). Valentine is like a Lindt Chocolate Truffle, except instead of chocolate, it's his alleged charm, and instead of a creamy filling, it's hatred and evil. Oh, wait. He's nothing like a Lindt Truffle… we're just hungry.
We're also told that Valentine is some sort of evil genius, but his repeated attempts to recruit Jace don't make any sense either. He doesn't even care about his own son, placing his plan first and foremost on his priority list. (1. Take over the world. 2. Clean bathroom. 3. Hang out with son and daughter.)
The only reason Valentine seems to interact with Jace is to do the evil-villain monologue thing and tell him his super-duper evil plot. Aside from that, we're not sure why he really needs Jace. Valentine seems perfectly capable of carrying out his plans without his son. Maybe he just wants to make sure his son isn't around to foil his evil plot… but if he didn't constantly grab Jace and tell him his plan, he wouldn't have to worry about that either.
But the biggest question, we think, is whether or not Valentine actually is Jace and Clary's father or not. Some characters think there's no resemblance between them at all. Others do. Valentine focuses on the similarities in his and Jace's personality, especially their worst qualities. "I admire your stubbornness. It's so much like mine" (2.13.59), he tells Jace. Is it genetic, or simply learned behavior?