सर्वस्य ही परीक्ष्यन्ते स्वभावो नेतरे गुणाः

sarvasya hi pariikShyantE svabhaavo nEtarE guNaaha

atiitya hi guNaan sarvaan svabhaavo muurdhani vartate 

सर्वस्य ही परीक्ष्यन्ते स्वभावो नेतरे गुणाः 

अतीत्य हि गुणान् सर्वान् स्वभावो मूर्धनि वर्तते 

Always examine the nature, never the other habits of a person

All habits and characteristics go down, while the nature always comes to the surface

This beautiful shloka is from a Panchatantra story called 'vriddha vyaaghrasya kathaa". It's about an old tiger and a greedy traveller. 

The same message that Vedaanta also teaches us. About understanding the nature of the beast. If you come across a three week old fawn, it would be okay if you fawn over it and pet it. Would you do the same with a three week old tiger cub? You might. But then you would be apprehensive of a protective mother lurking about. Whereas with the fawn, even an overprotective deer would be something you would be able to handle (at least in your mind). 

Once you know the nature of a person or beast, to expect it to go against that nature is stupid.