In Abhodah Zarah (26b, Tosephoth) it says:
"Even the best of the Goim should be killed"
The Schulchan Arukh, after the words of Iore Dea (158, 1) that those of the Akum who do no harm to Jews are not to be killed, namely those who do not wage war against Israel, thus explains the word Milchamah—war:
"But in time of war the Akum are to be killed, for it is written: 'The good among the Akum deserve to be killed, etc.' "
V. A JEW WHO KILLS A CHRISTIAN COMMITS NO SIN, BUT OFFERS AN ACCEPTABLE SACRIFICE TO GOD
In Sepher Or Israel (177b) it says:
"Take the life of the Kliphoth and kill them, and you will please God the same as one who offers incense to Him."
And in Ialkut Simoni (245c. n. 772) it says:
"Everyone who sheds the blood of the impious is as acceptable to God as he who offers a sacrifice to God."