Kindness

How is it one cannot trust another's kindness?  As my sister shared with me, "every human being is a combination of kind, mean, accepting and judgmental. It's called being human. Mother Teresa said something to the effect that she did what she did to make up for the Hitler in her heart! That you choose to behave kindly and keep other behaviors in check is a good thing."  Seems to me that you could trust a kind person more for protecting you from the cruelty of others and the mean-spirited thoughts in her own bleeding heart.  Wouldn't one be a greater friend by protecting you from your own cruel defensiveness?  Why wouldn't one trust that as a good thing?  I have a smart sister.  It is NEVER my intention to hurt others.