Meaning of "He who wants his dog killed has only to say he's mad."

I have to write a story about that proverb that uses it as a moral. Unfortunatly I can’t understand exactly what He who wants his dog killed has only to say he’s (the dog) mad means. Can someone explain it?

I would say it refers to making accusations that will be taken as true on their face, and acted on without investigating first – like accusing someone of being a witch in Salem.

This is only a guess.

You can get people to do something odious by misrepresenting it.

i.e. – if you falsely say a mad dog is around, other people will kill it because they wrongly beleive it is a threat to public safety.

By the same token, if you say something slanderous about d12, people might believe that too and kill Eminem or ignore him at the ice cream social, or something.

There is a very similar proverb in French - “Qui veut noyer son chien l’accuse de la rage.” - he who wants to drown his dog, accuses it of having rabies. (I believe “rage” in old-fashioned French also had connotations of madness)

In other words, if you want to get rid of someone, you can always find an excuse to do it.

I’ve heard of another similar proverb in English - “Give a dog a bad name, and hang him.” Anyone know if the meaning is the same?

Usually this crops up in conversation only partly stated, and becomes
“Give a dog a bad name…”
It can mean a couple of things, one is the way that reputations can live long after an event is forgotten, so the concluding unspoken part would then be,

“…and it sticks”

It can also mean that if you start something off badly, it can very almost impossible to correct, particularly with reputations.

The ‘hang him’ bit is hardly used but someone using the first part of this saying is very likely to have it misconstrued.