מסית-No Pity
In chapter thirteen of דברים, there are three separate cases brought in the Torah. The first is the case of the נביא שקר, the false prophet, whose goal was to turn people away from Hashem.
This is followed by the מסית, the one who incites others to worship עבודה זרה. This individual sounds very much like a missionary.
The final case is the עיר הנדחת, the condemned city, that is full of idol worshippers. It has no redeeming features whatsoever. The entire city is burned to the ground, and its inhabitants are killed by the sword.
It is interesting that only in the case of the מסית, does the Torah say, ולא תחוס עינך עליו, that your eye should have no pity on him.
The מסית is viewed as a cancer within the society. The full extent of the law shall be applied to him. The Gemara in סנהדרין says that the toughest judges should be appointed to deal with the מסית.
A tough judge would be one who is either childless or a grumpy old man. These would be no nonsense judges who would have no patience with the מסית, and he would likely receive the death penalty.
There are times when compassion is necessary, while there are others, where compassion is actually cruelty.