Rob Kardashian thinks Kim Kardashian West is ''ruthless''.

The 28-year-old sock designer compared his older sister to the ''bitch from 'Gone Girl''' in a recent Instagram post and insiders say he sees similarities between the psychotic schemer and the brunette beauty - who is married to Kanye West - because he thinks her marriage is a ''mutually beneficial business arrangement'' and she never lets anyone get in the way of her plans.

A source said: ''Rob feels her ambition borders on ruthless and has long disagreed with the way she sells every inch of her life to the highest bidder.

''He also seems to think her relationship with Kanye is more of a mutually beneficial business arrangement than a marriage.

''Like the 'Gone Girl' character, Rob feels Kim is just playing the role of the perfect wife to the media, when the reality is nothing like that.''

Kim is said to feel ''mortified'' by Rob's public outburst but the troubled star - who skipped his sister's wedding to Kanye last May - is reportedly threatening to go even further.

The source added to Grazia magazine: ''Kim feels mortified that he'd attack her on such a public forum.

''She's always taken a tough love approach to him, but communication between them has totally broken down.

''He's threatening to do a tell-all where he lifts the lid on life with the Kardashians from the inside.

''He knows where the bodies are buried and isn't interested in towing the family line.''