Were they insanely brave or simply insane? I don't know.
There are four ways to go for the old-school Asian warriors:
1 — go to battle and win
2 — die in battle
3 — go to battle, realize you’re beaten, and run for it (still honorable, but you don’t brag about it)
4 — dishonorable options are surrender and capture
about 99% of Japanese on Iwo died there (and good riddance).
Regarding option #3 (and #2), Chinese military thinker Sun Tsu noted that it was necessary to always leave the enemy an avenue of escape, otherwise the adversaries would fight to the death.
Pagan fanatics is what they were.
Surrender in battle was considered a terrible dishonor. Worse than rape or murder.