I think the plan was that KM would use them to disguise a vehicle he would employ in acts to make his estranged wife disappear. But, I think KM lost his nerve after JFD's violent murder. I think that when plans moved to execution, the reality of what they were doing (and the likelihood that they'd all be caught) hit him like a Mack truck and he wanted nothing more to do with continuing the plans. So, he freaked out and destroyed his cellphone in the hope that by doing that LE would not be able to find the communications he had with FD close in time to before and after JFD's disappearance. (ETA: And he decided NOT to pick up the doctored license plates from their hiding place in the storm drain.)
FD may have been and MT may be evil enough to play it cool and above-it-all despite JFD's family's money in FD's accounts, JFD's blood on their hands, and her murder FOREVER on their souls, but I think KM - as weasel-like as he was and may still be - retained a touch more humanity than they did. He may have been okay with helping FD take JFD's family's money and he may have been okay with discussing plans before they became reality, but I think JFD's murder horrified him and he likely regrets the day he ever became acquainted with FD and his girlfriend.
I could be wrong. But, I suspect LE knew there was some raw material to work with (conscience, fear of losing his reputation among family who mattered to him, horror at crossing over the boundary into violent murder... some remnant of humanity FD and MT lacked) in trying to get KM to serve as witness against FD and MT. If so, I hope he rises to the occasion.