Well if you all remember, Craig and Kelly's initial story was that Melissa OD'ed and Craig and Kelly panicked. The latest version, as heard on BNR last night has Craig killing Melissa in an effort to save Kelly from serious bodily injury or death.
Yeah, I could see how a 5'9 240 lb. Macho Muscle Machine would be unable to defend his wife without killing a 110 lb girl in the process. Apparently Melissa had been studying under Randy Couture at Team Quest and had Kelly in a triangle choke when Craig showed up and saved the day.
Totally plausible explanation.