How does buying her a plain ticket and hotel room establish an alibi? She was going home to see her Mother regardless.
If you hit her over the head to kill her and the first strike does not do it you
A. hit her again
B. force drugs down her throat
" OK Kelly, I'm going to hit her over the head once and if it doesn't kill her be ready with the Nubain and duct tape"
If your willing to hit her once then your willing to hit her as many times it takes.
There is no evidence that shows she was still alive when the car was set on fire. No evidence of blood at Craig and Kelly's home. (so far)
If Melissa did steal from them they could have
A. Fire her and kick her out of your house for good.
B. kill her
C. buy her a plain ticket and two nights at a hotel room and have one more f%ck session before you kill her.
Killing her does not solve the problem. If they were out money they certainly are not getting it back if they kill her.