I'll be honest with you...I'm not convinced it wasn't him... exactly like his parents explained it to him. He blacked out, snapped, and made the ultimate mistake.

We only ever get Veronica's theory about Aaron. (And his line about not letting a 17 year old P-O-A ruin his life) Sure, him trying to kill Veronica over the tapes is suspicious, but regardless of his motive for doing that, they were still sex tapes of him with the murdered underage girlfriend of his son. That's never gonna look good, no matter how you look at it. And him getting Kendall to source Duncan's hair is also shady but if you were afraid you were about to go to prison for a crime you didn't commit and you had the means to plant false evidence that sheds doubt on the narrative, wouldn't you take every opportunity?

Duncan having Aaron killed later doesn't really feel like "justice" as much as it feels like "tying up loose ends" to me...but that's just my opinion.