He thought he was protecting his wife

My take on the ending is that he believes his wife, being a Green-Eyed Monster (green w/envy), having seen the pictures of the girls he took as he explored the island, killed the girls.
He then being a brow beaten husband, who has spent the evening being worn down and fed all the details of the crime scenes decides to protect his wife who he never stopped loving.

"I can't believe she'd go to these lengths to prove a point." Then spontaneous confession.
The lengths he thinks she'd gone to is to kill any pretty young thing he might have his eyes on.

Anyway I think this is a simpler and more reasonable answer than the conspiracy of her hiring a killer.