Hospital, drug store and coffee shop cameras helped to spot him. The argument is HIS version of events, LE isn't necessary buying that. Perhaps he was trying to steal prescription pad (from Mike Newell from Philadelphia Inquirer, speaking on the phone). No criminal past other than DUI. He has a prescription drug addiction (to pain killers). Perp told this to LE. HLN
Whatever the diminutive doctor said to him, this murdering lunatic likely took as an insult, whether the insult was only an imagined or perceived slight due to his own projections and warped life views.
Some people are prone towards misunderstanding others because of their own preconceived notions about who the other person is, and their own stations in life. Some people go around with huge chips of their shoulders. They feel entitled and walk about the world with anger and hate. I think this violent psychopath already had tremendous insecurities and self-esteem issues.
I would think that his living in a household full of women in a home he does not own but rather is merely an at-will tenant of his gf and her mom's home, that he felt extremely emasculated. He likely butt heads with the women quite often over who's the boss and master of their domain. And obviously the women would win out because it was their home that he had moved into. So when he saw this doctor living in her own house with her dog, he likely hated her on sight, his jealousy, anger, and machismo attitudes raging.
Then when she was giving him instructions about how to do his job (maybe she pointed out where she thought he should spray as that's where she saw the rat), his pent-up anger and rage over his own living situation and powerlessness towards the women in his life, likely caused him to flip. So he took out his rage and misogynist hatred on poor Melissa.
But I also think the murder was premeditated. Why would an exterminator have masking tape and rope, accelerant, and a lighter? Also, he stalked her for many blocks. Why would an exterminator stalk a complete stranger on the street? I think he must have done some google search on her before he met her on Monday. He knew who she was and how she looked. Otherwise, he would have just been stalking a random woman, not the very client whose home he was scheduled to perform extermination service.
Regardless, I hope he gets his just deserts -- exterminated.