Does anyone know why Christianity was spread by violent means?
My answer would be that it was because Christianity was useful to emperors and kings. It says that kings are divinely appointed by god, and that it is sin not to submit to them:
"Let everyone be subject to the governing authorities, for there is no authority except that which God has established. The authorities that exist have been established by God. Consequently, whoever rebels against the authority is rebelling against what God has instituted, and those who do so will bring judgment on themselves."- Romans 13:1-2
"Remind them to be submissive to rulers and authorities, to be obedient" - Titus 3:1
Also, the Sermon on the Mount and related scriptures define a way to live and view one's lot that has also got to be very attractive to anybody trying to exploit you and keep you from rising up. Blessed are the meek. Turn the other cheek if smitten. Be content with your poverty, for your reward will come later in an afterlife where you will be an equal and loved by God for your submissiveness.
It seems likely that Constantine understood that this religion would benefit him, made it the state religion, and began to use the might of his government to promote it.
By the time of the Crusades, the Conquistadores, and the Inquisition, the motives for spreading Christianity using force and violence had probably evolved to more that just making more people Christians. Now, accumulating and consolidating wealth and power were probably instrumental as well.
Here are a few more views on the usefulness of religion to rulers, and the incentive that they have to impose it on others:
"Religion is regarded by the common people as true, by the wise as false, and by the rulers as useful." -Seneca the Younger
"How can you have order in a state without religion? For, when one man is dying of hunger near another who is ill of surfeit, he cannot resign himself to this difference unless there is an authority which declares 'God wills it thus.' Religion is excellent stuff for keeping common people quiet. Religion is what keeps the poor from murdering the rich." - Napoleon Bonaparte
"If you want to control a population and keep them passive ... give them a god to worship" ~ Noam Chomsky