Was he planning to commit suicide?
Was he inteded to his step-son's well-being?
Was "Rosy Hills" a name of a property?
Did the step-son misunderstand everything?
At the last hospital scene, he sounds sincere.
Or was it also a part of his scam?
I would answer yes to your first four questions. That was part of the story's twist. There was no scam on part of the stepfather.
I took it he and mommy dearest were planning to kill the son! Remember, he says "take care of Harper and then we can be together forever."
That "justifies" Harper* killing the stepdad after all, and is why the son doesn't keep the exonerating video recording... except wait, the son didn't hear the tape the nurse put next to the mom. So... ?
I did take it that Rosy Hills was a property & thus a legitimate business trip.
Re-reading my first sentence, I now see your basis for stepdad committing suicide.
But that's OK... I love ambiguity!