There's no way the P would be totally uninterested in how RA did what he did. They do have solid evidence showing that and it's why they arrested and charged him for murders to begin with IMO. I believe it was something found during the SW of RA's home maybe in connection with a recent tip in.
I can't say if we or anybody other than RA will ever truly know the why? Maybe during his 20+ confessions he did state why, IDK.
IMO that's why the D has been so obsessed with getting the SW tossed, there's something very incriminating there. If RA is completely, 100% innocent like they've stated from the beginning, there wouldn't have been anything incriminating to be found in those searches. Why file 4 Franks Motions?
ALL JMO