"Mad, Bad, and Dangerous to Know"
Noah Cross is "mad, bad, and dangerous to know" (which is 1) a sick burn and 2) something Lady Caroline Lamb said about the poet Lord Byron. Shmoop: Feeding You Lit Nerd Insults Since 2009.).
Cross is mad and bad enough to have sex with his own daughter. And he doesn't stop there: he murders that same daughter's husband and steals the water supply from the L.A. reservoirs, sending it to land that he owns so that he can sell it back to the city. He wants to make all the water his own private property instead of a public utility—which is the way things used to be before the company had to go public.
Basically, he's not a nice guy.
But when Jake first encounters Noah, he doesn't meet someone who matches this description. Cross seems like a gentlemanly old dude, extremely rich but not necessarily malevolent—he expresses real concern and urges Jake to track down Hollis's mistress, saying that she'll be able to reveal the truth (ironic, considering that this "mistress" is really Cross's own daughter/granddaughter, and that he wants her for himself.)
He puts on an act, convincingly playing the part of a concerned father to Evelyn, when he's really her abuser and eventually the author of her destruction. Note: don't trust a guy just because he takes you to a nice fish dinner and looks slick in suspenders.
Bad Guys Finish First
When Jake finally realizes that Noah's trying to steal the water for his own profit and that he killed Hollis by drowning him in the saltwater pool in back of Evelyn's house, he confronts him at the very scene of the crime. And like a flower turning into a (malevolent) dove under a magician's wand, Cross metamorphoses into a totally menacing presence.
Check out how cold-blooded Cross turns out to be:
GITTES: I want to know what you're worth—over ten million?
CROSS: Oh, my, yes.
GITTES: Then why are you doing it? How much better can you eat? What can you buy that you can't already afford?
CROSS: The future, Mr. Gittes—the future.
What. A. Sociopath.
Cross wants control and power for their own sake—he wants to dominate people. And not just people that are currently alive—he wants to dominate future generations. At this point, money is a secondary concern.
Oh, and as for that whole "murder/incest" thing:
CROSS: You see, Mr. Gittes, most people never have to face the fact that at the right time and right place, they're capable of anything.
That might not be true (please, for the sake of our faith in humanity, let that not be true), but that's how Cross sees things: he's utterly reptilian. Part of the reason he's able to keep his cool is because he totally lacks the pangs of guilt and moral scruples that other people experience. At this point, he's willing to exercise raw power and trespass against the social order in the most aggressively taboo ways.
In the end, the movie suggests that monsters like Cross thrive and win. The cops won't listen to Gittes when he tells them what Cross is trying to do, and they shoot Evelyn after she (non-fatally) shoots Cross.
After Evelyn dies, we see Cross pull his granddaughter/daughter, Katherine Cross, out of the car. He's gained possession, and there's perhaps a nasty implication that he'll do the same thing to Katherine that he did to Evelyn. His career of violent domination and control isn't over yet.
At the end of Chinatown, evil is triumphant. As they say, "Nice guys finish last"—and Cross is heading for the finish line.