Robert Spencer, who is one of the leading Christian authors on Islam has a fascinating insight into the crusades in this video - The Crusades, Fact & Truth.
The Crusades have a bad reputation, because, as Spencer explains, Westerners have a healthy attitude of self-criticism. Such self-criticism is completely absent in Islam. Spencer points out that it was the invasion of the Turks that began the Crusades.
While it is true that many of the Crusaders did evil acts, the Crusades in themselves were a defensive measure and not an offensive one. Should Christians be ashamed of those Crusaders who commited evil? Sure. Should Christians be ashamed of the Crusades themselves? Never.
When soldiers came to John the Baptist asking him what to do, he didn't tell them to put their swords away, but
"Rob no one by violence or by false accusation, and be content with
your wages." (Luke 3:14)
The Bible also says that there is
"a time to kill, and a time to heal; a time to break down, and a time
to build up" (Ecclesiastes 3:3)
Often, Jesus statement of "turning the other cheek" is quoted to suggest that any act of self defense is against the teachings of Christ. But I answer that somebody slapping you on the cheek doesn't kill you, whereas if someone tries to kill your body, you are obliged to defend yourself, because your body is a temple of the Holy Spirit (1 Cor 6:19).
"If any one destroys God's temple, God will destroy him. For God's
temple is holy, and that temple you are." (1 Cor 3:17)
The implication of this statement is that if you allow your body to be destroyed, then you shall be held accountable for it. Jesus also said,
"But now, let him who has a purse take it, and likewise a bag. And let
him who has no sword sell his mantle and buy one." (Luke 22:36)
Paul says that rulers are servants of God to execute God's wrath on the wrongdoer.
"But if you do wrong, be afraid, for he does not bear the sword in
vain; he is the servant of God to execute his wrath on the wrongdoer."
The Bible on many occasions praises soldiers. One of the first gentile converts to Christianity was a Roman centurion, Cornelius (Acts 10) and we don't see any demand being made on him to leave his position as a centurion. On another occasion, Jesus marveled at the faith of another centurion (Matthew 8:10). I therefore do not see any reason why a Christian cannot serve in the army.
Had the Crusades not taken place, the entire western world might have been under subjection to Islam today. As a Christian, I shall never be ashamed of the Crusades. Many Christians gave their lives for saving their brethren.