IMO, she was specifically targeted (nothing random about her encounter with the SG as they knew each other). If the SG harbored anger towards her for his termination (we still don't know details of employment), he may have wanted to harm her (or her family members), but not necessarily be a murderer and selling into the world of trafficking would benefit him financially while seeking revenge. IMO, he suspected almost immediately that a witness may have been able to identify him as being with her last so he returned the vehicle to her apartment to cover his tracks.
so in what you have here, how do you figure the security guard who was an acquaintance in this? They have searched his home 2 times and taken evidence to test, including 2 cars and a mattress.