If someone is attacking you then you may kill or disable him. Self-defense is a biblical principle. However, you may not toy with him as with a spider dangled over a candle’s flame. Sadism is not a biblical ideal for Christian practice. All men—even captured soldiers in aggressing armies, criminals, and terrorists—are created in the image of God and must be respected.
Thus, captured soldiers should not be tortured. Once an aggressor is captured then he is no longer a threat. He may be executed when doing so is the just penalty for his crimes but he must not be tortured. Do unto him as you would have him do unto you if you were the one captured. As well, remember that most soldiers in aggressive actions are conscripted by states and may not share the philosophical goals of their rulers. They may not want to fight but are doing so to save their lives from state tyranny. This fact should at least be a mitigating circumstance in many cases that gives us further reason to shun the practice of torturing captives.