luke skywalking

tl;dr: Oliver is telling a lie the entire movie, of course he didn't plan to kill the Cattons from the start but he wants you to believe that so badly, and most of all he wants to believe it himself.

1. Elspeth

While Elspeth might be the only other actual person in the room with Oliver, I think it's highly unlikely that she was the one Oliver was lying to, concidering that she is literally in a coma and about to be murdered by Oli. So why would it matter anymore what she thought of him?

That brings us to the next person he could be lying to:

2. Himself

The only person who is really hearing Oliver's story is Oliver himself. And that could be exactly who he is lying to.

He tells himself that his goal had always been to get the house instead of his goal being Felix's attention.

Because he failed so misrably at his first goal he now wants go make mostly himself believe that he never had that goal in first place. He tells himself that he always wanted the house, the money, Saltburn. Because recognising the rejection would mean recognising himself as the loser who doesn't fit in and that would just hurt too much.

But there is also a third "person" in the room with Elspeth and Oli that Oliver could be lying to:

3. The Audience

During the entire movie Oliver desperatly wants to fit in and belong, be a fixed part at Saltburn. But he never achieves that goal, he is always on the outside looking in.

What he can be a part of in the end though is the story itself.

In storytelling there are at least 4 major character types:

the protagonist

the antagonist

the confidant

the love interest

Next to Felix (or anyone in the Catton family, for that matter), Oli could never imagine or dare to imagine that he is the protagonist. How could he see himself as such, concidering that he never felt like he belonged or fit in?

Neither could he be the confidant as... literally no one really confides in him.

That leaves the love interest and the antagonist. The very first thing Oli states in the movie is that he loved Felix but he wasn't in love with Felix. He is purposefully removing himself from the role of the love interest. That is not what he wants to be seen as, even though it is so blatantly obvious that he is in love with Felix.

A reason why he denies being the love interest is because he simply failed at being such. Felix never saw him that way, they were never a thing, quite literally he was never Felix love interest.

That only leaves one character type: The antagonist. The moment Oliver realises that he is never going to be the love interest he decides that he is going to be the last remaining role he could possibly play: the antagonist. And thus he kills Felix.