To simply say "it's ok to kill your enemies" is an attempt to place the situation in the worst possible light. The use of the word "ok" trivializes the matter. Even when it is necessary it is still not "ok". It is however permissible when protecting your life or others. Taking another's life should never be taken lightly or trivialized. It should be a last resort. Even then it is a sad and sorrowful thing to do.