the next day when he was driving his train. >> they moved north and then back south along the railroad. >> reporter: but now prosecutors had to tackle the head scratching question of motive. why would tony kill melissa in the first place? debra villegas testified that tony became very jealous and angry with melissa because she'd virtually replaced him in the household. >> was melissa helping you through this difficult time in your life now, through this divorce? >> yes, ma'am. >> reporter: and that was the theory. in order to get back at debra, he killed her best friend. >> you have these domestic homicides all the time but this had a twist to it because he didn't kill debra. >> reporter: then tony and debra's 23-year old son, caleb, was called to testify against his father. the usual blank stare on tony's courtroom face changed as his son recalled how his father blamed melissa for the divorce. >> did he tell you that it was