Every time I rewatch Saltburn, I am reminded of how well EF understands shitty rich people.
All of the Cattons are awful, but they're also rich and pretty and flying high on the myth of their own Specialness, to the degree that they get so much of the audience believing in that myth alongside them.
Felix isn't kind. He's a shallow, narcissistic, possessive user just like his mom. He expects the world to revolve around him; when it doesn't, he simply cuts out whatever part of his world displeased him. The most frightening thing Felix can encounter is a loss of complete control over the people around him - exempting his parents, who are where his wealth and power stem from and thus are the sole parts of his world who are allowed any measure of control over him.
Venetia isn't insightful. She's a mess. She's nasty, jealous, and just as shallow as the rest of her family. She covers up her own insecurity with reflexive cruelty towards anyone who reminds her that she will always come second to Felix in the heirarchy of her family's mythologized Importance, but she can't break away from the very family that destroys her because she sees her place in that family as also the only source of Importance she does have - lesser though it may be when compared to Golden Boy Felix. Even in her final conversation with Oliver, however, he sees her more than she sees him. Her rant is a scattershot mess of mutually contradictory insults - he's harmless, he's got nothing to do with their family, he's obsessed, he's a moth batting at the windows who can't get in, but he's also the creep lingering in the corners, the hungry beast who ate Felix up and licked the plate clean, the reason she's laughing and the freak she can't stand - and he lets her talk herself out, and then he kisses her - because he's the freak who got under her skin, the freak who saw her freak and made her come like no boy ever has in her life, the freak who loved and hated her brother as obsessively as she did, the freak who's wearing Felix's cologne and his bathrobe and whose kiss she's still desperate to have.
Oliver saw them in ways they couldn't bear to be seen - as shallow, as selfish, as flawed, as people who could be used back when they used him - and it scared the hell out of them. But they were too high on the myth of their own untouchability that they couldn't let themselves be as scared as they should have been.
Felix still drank from the bottle Oliver handed him even right after admitting Oliver made his blood run cold with fear. Venetia lounged in that tub and let Oliver prowl around her like a hungry panther while she called him a moth. And the only family member who didn't believe that Felix and Venentia killed themselves - one ostensibly a victim of a drug and alcohol OD, one ostensibly a victim of her mental health issues - was Farleigh. Farleigh, who was thrown under the bus of his family's blatant racism right after all his boasting about how Saltburn was his house where he belonged.
And all of this is all the more impressive when considering that EF didn't even set out to write Saltburn as class commentary. She just wanted to write something fun and ridiculous about boys.