If an enemy kills an enemy, there is nothing to excite pity either in the act or the intention,—except so far as the suffering in itself is pitiful.
 Aristotle, Poetics (c. 335 BC). copy citation

add
Author Aristotle
Source Poetics
Topic pity killing
Date c. 335 BC
Language English
Reference
Note Translated by S. H. Butcher
Weblink http://www.gutenberg.org/files/1974/1974-h/1974-h.htm

Context

“Let us then determine what are the circumstances which strike us as terrible or pitiful. Actions capable of this effect must happen between persons who are either friends or enemies or indifferent to one another. If an enemy kills an enemy, there is nothing to excite pity either in the act or the intention,—except so far as the suffering in itself is pitiful. So again with indifferent persons. But when the tragic incident occurs between those who are near or dear to one another—if, for example, a brother kills, or intends to kill, a brother, a son his father, a mother her son, a son his mother, or any other deed of the kind is done—these are the situations to be looked for by the poet.” source