Public execution is the only remedy. It's important to limit these idiots' ability to pollute the gene pool any further.
In this instance, I would not be opposed to hanging. Some will argue that their crime wasn't 'that bad', but make no mistake: the type of person who'd commit *this* sort of crime is the type of person who'd do *anything*.
If it were a couple of 11- or 12-year-old jiggaboos, I might be able to give them a slight pass, but the one animal is 21, and his homie is certainly older than he, so we're talking about seriously flawed DNA.