Why goodness and kindness must be God based

A man filled with great qualities, once came to the Alter Rebbe and asked “what I’m lacking?” and he replied (I may be mixing two stories, but the punchline is accurate) “goodness and kindness musn’t come from a good heart, rather because God tells us to” the man fainted, later they asked him why he fainted, he said, there is a bird called a chasidah (kind) because it is benevolent – realizing, that all my good heartedness was no more worthier than a bird, I fainted.

On the face of it, it makes no sense – if I happen to have a good heart, compassion running through my blood, what’s wrong if I simply exercise it – but don’t you think all evil people also have or had goods hearts, don’t you think they instinctively prefer love to hate.