Couple of things (well, three).
Firstly, why do people who despise violence (such as was committed on this poor animal) always deplore said violence and then want to do the same violence back on the original offenders!? To me that seems a bit hypocritical. Sounds to me like they don't object to violence so much, they just want public approval for their own violence. Where's the morals in that? America has capital punishement, and still has violent crime - capital punishment clearly doesn't work as a deterent.
Secondly, when were these 'good old days'? Was it as early as the 60's, when the Moors murderers killed several children? Or was it in the Victorian age, when animal fighting and child labour were rife?
Lastly, to all those who say,'a clip around the ears never did me any harm'. Yes it did - it turned you into someone who thinks committing violence on others is acceptable. All violent punishment teaches us is that you get your own way using violence. And whatever that is, it most certainly is not respect for other people or responsibility for one's own actions.
No one would suggest that the people who killed this animal shouldn't be punished. Of course they should. But it's better to get at the root cause to prevent it happening again than perpetuate the violence.