From the Tanakh
To everything there is a season, and a time to every purpose under the heaven. A time to be born and a time to die; a time to plant and a time to pluck up that which is planted; a time to kill and a time to heal; a time to break down and a time to build up; a time to weep and a time to dance; a time to cast stones and a time to gather stones together; a time to embrace and a time to refrain from embracing; a time to seek and a time to lose; a time to keep and a time to cast away; a time to rend and a time to sew; a time to keep silent and a time to speak; a time to love a time to hate; a time for war and a time for peace.(Eccles. 3:1-8)
From Or HaRa'ayon
Mercy toward the cruel is not a good trait. Quite the opposite, one is duty-bound to separate oneself from the evildoer, even if this is a difficult step, and even if it appears cruel. The cruel, wicked person will influence goodness and corrupt it. There can be no coexistence between evil and upright people-only separation.
The mitzvah of eradicating evil from our midst requires us to hate it, as in Psalms 97:10, quoted above, "Those that love the L-rd, hate evil." It is the duty of him that loves G-d to hate evil and evildoers, for they are G-d's enemies. Nonetheless, in the alien Hellenist culture, the themes of love and hatred have been so entirely distorted that it is a terrible crime to speak of hatred as a halachic duty in the right time and place. False love finds a hundred different ways to overlook evil. Advocates of that culture have transformed all such traits as cruelty and revenge into an evil that must be shunned. Such is not the Torah's way... (Or HaRa'ayon p.148)