I know that suicide is prohibited in Islam. What I want to know is if it is allowed in certain circumstances? I can think of two such circumstances: a soldier who has secrets of his country in his heart kills himself so that he is not caught by the enemy as his capture can cause great harm to his country. Another such instance is a girl ending her own life if someone approaches her with ill-intentions.
Suicide should be avoided in all circumstances since it is equivalent to rejecting the basic scheme of the Almighty according to which he has created man to test him through good and evil circumstances1. One should also remember that the Almighty has specifically mentioned in the Qur’ān that He never burdens a person with a responsibility he cannot bear2 .So however tough be the circumstances, a person should boldly face them knowing that they have been ordained for Him by the Almighty, and that he has also been equipped with the required resilience by the Almighty to face them.
As regards the first situation you have mentioned, I think that instead of ending his life the soldier should wait to see what his enemies do with him. If he is forced to reveal state secrets or even threatened with death, he can refuse and even let them take his life. In this way, he will die the death of a martyr and shall duly be rewarded in the Hereafter.
In the second case also, the lady should not end her life. She should boldly face the consequences of the heinous deed. I know that this is easier said than done, and that it is very difficult to bear the aftermath of such an incident, but then this is a form of trial and test from the Almighty. By patiently bearing such a great trial, a lady can earn for herself a very high reward in the Hereafter.
This is what my understanding of Islam says. However, if in the two situations that you have described the soldier and the girl do commit suicide, the Almighty might treat them leniently considering the compelling circumstances. Nevertheless, this is what the Almighty might to do. His servants should always try to go through the test they have been put through -- however severe it may be.
1. We will test you with good and evil circumstances. To Us you shall be recalled. (21:35)
2. Allah does not burden a soul with more than it can bear. (2:285)