Even though he's already planning the murder, the obvious one between Hayden Danziger and Rosanna Wells is when he hits her, something a blackmail victim should never do to the blackmailer unless they're all set on murdering them right then and there!

I do have one big problem with that scene, and it's about the Rosanna line that actually causes it to happen. She simply tells him a little sarcastically to "be sensible." But a minute earlier when he says that "No two-bit singer" (or something like that) is going to jeopardize his business, she smiles and says "This one is."
It seems like THAT would be the line that makes him loses his cool, instead of the other one.

One thing about the Bye Bye Sky High scene is that Oliver professes to have really loved Bertie, so you would think he would have been a bit more kind to Bertie considering that it would be his last moments alive.

One thing that always seemed odd about Oliver's preparation for the murder is that when he is setting up the umbrella upstairs prior to the murder, he is almost giddy, presumably with how clever his set up is. However, I find it hard to fathom that anyone who is about to murder their closest friend, or even the fact that they are about to cross the line into an unthinkable act no matter who the victim, would not be giddy - they would be nervous, contemplative, distressed, but not giddy.