How could it be possible for the killing crusaders of the past do what they did when they believed in Jesus the Son of God their savior?

Far too many Christians convinced themselves that it was their duty to make the Holy Land Christian.

When the bible was written and the early Christians were teaching peace and love, Islam had not yet been invented. Islam grew and in the 7th or 8th century, there was an event called the Arab invasion. They stormed out of the desert conquering and killing and forcing survivors to convert from Christianity to the new religion of Islam. Here is a short video showing the area of their conquest versus the area of the crusades.

https://www.youtube.com/watch?v=I_To-cV94Bo

The crusades happened because of the islamic jihad that conquered to the outskirts of Vienna. Charlemagne and his forces managed to drive them off but they could not dislodge the muslims from Christian Spain for 800 years. When Ferdinand and Isabella united their kingdoms of Castille and Aragon, they and their allies managed to send Islam packing back to where they came from. That was in 1492. The crusades were fought for survival of Christianity against a fierce and warlike enemy. 

edit;

Christianity did not begin with the reformation mr. anonymous. It had gone for over 1500 years before.

Carnal warfare is sinful and condemned in 2 Cor. 10:4.

The question should really be, “Why did the Muslims invade Christian lands to the point Christians had to mount a defense or be destroyed?” If you don’t think God can assemble armies to be his hands and feet then read the Old Testament

To understand Christianity a cram course on Saint Paul need be researched 

Romans 3/7 " For if the truth of god hath more abounded  through my lie unto his glory , why yet am I also judged as a sinner .

Conveyance to me was Paul saying that if he lied to someone to maybe get them to church and they got saved then the lie was really something sinless then .

Or that anything goes if it is for gods' glory .

All under pretense that GOD is on their side .

The reason Zionism exist is because in Revelation   Jesus clearly states that If the second Jewish temple is not rebuilt exactly on the very foundation as the second tore down Jewish temple he can not  return . Some 19 acre plateau exactly where the dome of the Rock stands  today  . The Spot where Muhammad ascended to heaven to receive the Quran .  2nd most holy Muslim site .

Even back then they wanted that 19 acres      for that . 

19 acre vertex of nuclear focus  OVER A MYTH 

maybe they werent really christians

You don't seem to understand that 'civilizing' soulless barbarians who destroy women and children without a thought, poison entire communities for fun and will die to the last man defending tribalism over brotherhood is a very messy but noble business.

Jesus Himself calls us to deadly arms in Luke 22:35-38, giving us the famous 'two in twelve' proportion for deadly arms required to protect the brethren while living under 'secular' rule, (later becoming the basis of the holy knighthood and the American 'right to bear arms'), while reminding us that He did not come to bring peace to the  world but a sword of justice that would ultimately bring us the global integration, cross-tribal brotherhood, liberty and spectacular prosperity we enjoy today.

The 'Crusades' and other missions into the dark pagan worlds of previously inextricable evil were crucial to accomplishing what people like you now take for granted. Believe it. It is quite real. 

Those who started the crusades did not heed Jesus' words seriously. They interpreted the Bible incorrectly. Were they true Christians? Only God can know.

History is full of people who made mistakes. Humans do err because of the fleshly nature.

God have mercy.

Good people do good things and bad people do bad things but religion makes good people do bad things.

The church wanted more power and money, the crusades were designed to get them that. Simple. Power and/or wealth are pretty much the reasons for every war in history when you get into it