Why were so many japanese soldiers killed in battle of owl him a instead of being taken prisoner

Many Japanese soldiers were killed in battle of Iwo Jima because:

1) Their shinobi code of honor: They had a some 'rules' they had to follow. One of which was that they could not be captured. If they were wounded, they would have to suicide, and it was particularly better to take their own life when the enemies were around.
2) Because of this, they did not have respect for any Allied soldiers that surrendered. This made the Allies wary of the Japanese, and fought the Japanese with a different "rule" then their German counterparts. In fact, some soldiers adopted the motto "shoot before you ask questions", and it was used widely in battles. This motto meant that they would accept no prisoners.
3) Iwo Jima was considered to be part of the Japanese homeland that was born out of the ocean when the Japanese god created the world. To them, this land was sacred, and they said & thought that these lands would never be conquered. This was part of the reason why many soldiers fought to the death, because they believed that their Islands would never be taken over by foreigners. 

fun fact: Kamikaze, meant divine wind, and it was widely used by the Japanese during the war, even on Iwo Jima. In the history before WW2, a group of Korean ships set sailed to attack Japan (back in the Samurai ages). However, they met strong winds and storms, and the attack force was destroyed. The Japanese named this kind of wind "kamikaze".

hope this helps

Primary sources are usually  more accurate and are written by someone who experienced it or witnessed it. secondary sources are those who heard of it after the fact that can lose details and or twist the event in a light that benefits them.