Well, just like humans, animals can be born evil due to how their mind is effected when it was born. However, they are few and very far inbetween so they are rare cases, as it is with humans.
It all begins on how they are brought up, were they cared for mentally and physically? Were they left to die and to fend for themselves? There are many factors to why animals end up reacting the way they. Even some tame animals can go bad due to owners treating them badly or training them to do harm like dog fighting.
Not all creatures will be nice, that is a given, but if you respect them and leave them be, more likely they won't do anything untoward.
Sadly in saying that, a poor plumber was attacked by 3 vicious dogs last week here in Australia, Owner was told to put them inside or have them away from the job site if they were nasty dogs. Well suffice to say the owner didn't do as instructed and the dogs mauled the man with horrible injuries. His ok and recovering in the hospital, however the 3 dogs that mauled him, were put down, even the owner's daughter was injured by them during the attack. It's been known now that these 3 dogs were terrorizing the neighbourhood for some time and the owner was warned if they didn't secure their dogs properly, they would be put down.
If raise animals with such care and love we may avoid these issues, but i'm not sure that will be enough to do it to be honest.
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