This is one of those stories you couldn't make up. Manchester housewife Tracy Seward was an Internet dominatrix; her fetish was "crushing". She had an international following, and when she decided to murder her lover, she had no difficulty in roping in a couple of suckers to do her dirty work. Philip Brierley was also the father of her three children, and there appears to have been no real motive other than wanting him out of the way so she could continue her affair with her affluent and much younger Swiss lover.
The victim was shot in the head with a gun that was smuggled from Switzerland. His body was then cut up, encased in concrete and dumped in the river. Seward and her helpers might have got away with it but for her lack of concern over his mysterious disappearance being a mite suspicious. There was also a lucky break in the form of an off-duty police officer who saw the body being dumped, although he didn't realise it at the time. Oh, and if you want to get away with murder, it's probably not a good idea to boast about it on the Internet.
This documentary covers the case in some depth, though it's best if you ignore the psychobabble.