>I always thought it was a tease - I think the killer knew he
>would never try to get any ransom - the note was just a cruel tease.

Yes. And ofcourse it is indeed possible to impose some 'cruel tease' on a randomly selected person or simply by whim. It is possible to hate the whole world, but I think the "cruel tease" implies a desire to cruely tease a particular victim.

He was not 'cruely teasing' the parents of 'that cute girl he had encountered'. He was cruely teasing the Ramseys and was doing it for some long simmering reason.