Heather was very attractive. That might explain why the husband strayed. But, what on earth would motivate him to cooperate with his wife in kidnapping and murdering her?
Fear of what she would do if he didn’t?
Cheaper and less painful than divorce?
Just a thought
See the "Texas Cadet Killers" case. Teenager Diane Zamora induced boyfriend David Graham to participate in the kidnap-murder of classmate Adrianne Jones, after Graham confessed to Zamora that he had had some kind of relationship with Ms. Jones.
IIRC there were some doubts about whether anything actually happened between Graham and Jones, other than he killed her to keep Zamora.