While I don't believe that from a Christian perspective Lewis is wrong in his statements here, I do think his wartime experiences tend to skew his exegesis of Christ's statements about loving one's enemies. Christ was primarily speaking of our personal enemies--people we can't get along with, people who intentionally oppose us and do us wrong--rather than enemies of the state during wartime. Those kinds of enemies--personal enemies--of course, Christians are not justified in killing, as the Bible makes plain (and with which Lewis would unquestionably agree). Quotes below are from the King James Version.
Proverbs 25:21-22: "If thine enemy be hungry, give him bread to eat; and if he be thirsty, give him water to drink: For thou shalt heap coals of fire upon his head, and the LORD shall reward thee."
Matthew 5:21-22: Christ said, "Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment: But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment: and whosoever shall say to his brother, Raca [i.e., "Worthless"], shall be in danger of the council: but whosoever shall say, Thou fool, shall be in danger of hell fire."
Matthew 5:38-45: Christ said, "Ye have heard that it hath been said, An eye for an eye, and a tooth for a tooth: But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also. And if any man will sue thee at the law, and take away thy coat, let him have thy cloke also. And whosoever shall compel thee to go a mile, go with him twain. Give to him that asketh thee, and from him that would borrow of thee turn not thou away. Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy. But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you; That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust."
Luke 6:27-36: Christ said, "But I say unto you which hear, Love your enemies, do good to them which hate you, Bless them that curse you, and pray for them which despitefully use you. And unto him that smiteth thee on the one cheek offer also the other; and him that taketh away thy cloke forbid not to take thy coat also. Give to every man that asketh of thee; and of him that taketh away thy goods ask them not again. And as ye would that men should do to you, do ye also to them likewise. For if ye love them which love you, what thank have ye? for sinners also love those that love them. And if ye do good to them which do good to you, what thank have ye? for sinners also do even the same. And if ye lend to them of whom ye hope to receive, what thank have ye? for sinners also lend to sinners, to receive as much again. But love ye your enemies, and do good, and lend, hoping for nothing again; and your reward shall be great, and ye shall be the children of the Highest: for he is kind unto the unthankful and to the evil. Be ye therefore merciful, as your Father also is merciful."