Humility, kindness, resignation, and all these spiritual attributes emanating from great physical strength are acceptable to God.
(‘Abdu’l-Bahá, Divine Philosophy, p. 98)
It is not advisable to show kindness to a person who is a tyrant, a traitor or a thief because kindness encourages him to become worse and does not awaken him. The more kindness you show to a liar the more he is apt to lie, for he thinks that you know not, while you do know, but extreme kindness keeps you from revealing your knowledge.
(‘Abdu’l-Bahá, Bahá’í World Faith, p. 412-413)
Strive ye then with all your heart to treat compassionately all humankind--except for those who have some selfish, private motive, or some disease of the soul. Kindness cannot be shown the tyrant, the deceiver, or the thief, because, far from awakening them to the error of their ways, it maketh them to continue in their perversity as before. No matter how much kindliness ye may expend upon the liar, he will but lie the more, for he believeth you to be deceived, while ye understand him but too well, and only remain silent out of your extreme compassion.
(‘Abdu’l-Bahá, Selections from the Writings of ‘Abdu’l-Bahá, p. 158)