If TM wasn't aware that SM called HRE from the payphone then HRE called his cell phone which was in TM's possession...you have the catalyst. From that moment on...things could have spiraled into a murderous rampage. TM would have been outraged knowing HRE was back in the picture after all the "preventative measures" taken by her to keep SM away. Big time motive.
what if SM didn't realize that TM was sending those extremely troubling photos to Heather ( and i'm not sure or just can't remember anymore if they were sent way before the murder meaning weeks, days as oppose to the night she was on her date) and realized it and got angry at TM making TM super angry and jealous, etc...she then throws him out of the car and he calls Heather from the phone booth...1) to apologize for the photos 2) to say he has had it and wants to leave his wife?
imo...she sent them from broadway at the beach that night, just to piss Heather off or at least to try too
oh my mind just won't stop...