Roy Unithorne despised his wife ...

Roy knew he was destined for a better life. It was Amy. Ugly Amy who kept him back. She must be disposed of finally and for ever; he set about making plans for the disposal slowly and systematically, making allowances, as he thought, for X - the Unknown Factor.

And there was Mrs Shiplake. She was also beginning to interfere with his plans. She too must be got rid of. After all - two murders were as easy as one.