I will attempt an explanation, but it has been a long time since I studied moral theology so I'd appreciate corrections and other ideas for examples.

This is a principle in Catholic moral theology that one can perform an action which has two effects, one good and one evil, under certain conditions.  Here is a list of the conditions from the Catholic Encyclopedia:

• The act itself must be morally good or at least indifferent.
• The agent may not positively will the bad effect but may permit it. If he could attain the good effect without the bad effect he should do so. The bad effect is sometimes said to be indirectly voluntary.
• The good effect must flow from the action at least as immediately (in the order of causality, though not necessarily in the order of time) as the bad effect. In other words the good effect must be produced directly by the action, not by the bad effect. Otherwise the agent would be using a bad means to a good end, which is never allowed.
• The good effect must be sufficiently desirable to compensate for the allowing of the bad effect

Here is an example from Captain America, (a movie I saw back when I still used to go to movies;))



Steve Rogers jumps on a grenade in order to save the lives of others, even though he expects to be killed.  He does not positively will his own death, but this is the only way available to him to save others.  The good effect of saving people comes directly from using his body to protect them, not from his death.  He expects many lives to be saved.

Contrast this with another scenario I've seen in movies.  A person is captured by the enemy and kills himself to avoid torture and possibly revealing critical information.  This is an example of using a bad means to a good end.  This person is willing his own death because this is a way to avoid torture.  Even though he has a good reason, that does not make his ѕυιcιdє morally permissible.

In Captain America the grenade is a dummy, but, if it had been live, Steve's death would not be considered the sin of ѕυιcιdє.  The torture-avoider has objectively committed ѕυιcιdє, although I'm not sure if it would be a mortal sin in those circuмstances.  At least, that is how I understand Catholic moral theology. 

Contrast this with another scenario I've seen in movies.  A person is captured by the enemy and kills himself to avoid torture and possibly revealing critical information.  This is an example of using a bad means to a good end.  This person is willing his own death because this is a way to avoid torture.  Even though he has a good reason, that does not make his ѕυιcιdє morally permissible.

I have always found this scenario a good springboard for studying moral dilemmas, and I have had to wonder if this might be morally permissible.  Stay with me on this.  The person foresees that he will become, in effect, a resource for the enemy if he is captured, a type of enemy combatant, if you will.  It is not the torture per se that he seeks to avoid, but rather being deprived of his free will and thus being induced to surrender information.  It would not even have to be extracted via torture.  The enemy could forcibly administer him a truth serum and gain the information that way.  Seen this way, as becoming basically an enemy combatant by force, I have wondered if he might be able to see himself as an unjust aggressor against his country, and be justified in removing any possibility of having this information extracted from him.  Seen this way, it might be no different from killing an enemy soldier.

I think at least a probable, even if less probable, case can be made for this.

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ѕυιcιdє is intrinsically immoral and cannot be justified for any reason. This is stated whenever similar questions come up in these sorts of books.

But would it be truly ѕυιcιdє?

The taking of another person's life is not intrinsically immoral, regrettable to be sure, but not evil in and of itself.  We may kill an unjust aggressor, viz. someone who is trying to kill us, and we may kill an enemy combatant in warfare.

My point here is that the captured soldier who has critical information in his mind, and who may be forced to reveal this information by being deprived of his senses, such as being tortured to the point of insanity, or as I said, being injected with a truth serum (or given some sort of narcotic or sedative) that will disconnect his free will from his powers of mind, could be thought of as having become an enemy combatant.  If the information is extracted from him, he has basically become the enemy of the cause that he defends, and the enemy of those who would be endangered by the divulging of this information.

Put another way, may one kill oneself if one is forced against their will to become a type of enemy combatant, and can only escape this by death?

I'm not attempting to dogmatize or challenge the Church's teaching against ѕυιcιdє, I just wonder if this is an angle that moralists have ever considered, and again, seen this way, whether it can properly be called "ѕυιcιdє".  Murder is intrinsically evil, but not all taking of another person's life is murder.  So might we be able to say that while ѕυιcιdє is intrinsically evil, not all taking of one's own life is ѕυιcιdє?

To return to the example given, if a soldier jumps on a grenade intended for his fellows, yes, he can be said to have taken his own life, but he cannot properly said to have committed ѕυιcιdє.  How is that different from taking his own life to save his fellows by insuring that classified information dies with him?