[gkruCRi1]

You can practice Metta meditation as shown in this youtube video. http://www.youtube.com/watch?v=g3_lqd4Sgfc.

I personally think it's helpful to look at why they might've done this. You said they did it solely for amusement. I doubt this is the whole truth. People who do things like this have usually suffered a lot already in their life. They could've been victims in the past, people do what they know. The action of animal cruelty also shows that they have no understanding of what will actually help themselves and make them happy. They're misguided and you should pity them.