Now I think understand, thank you. I wonder if the catholic morality would consider the concept of “self-defence” literally - defending yourself? Or does it also include the defence of others? Maybe the defence of one’s children, or the defence of my neighbor’s children, or maybe even the neighbors themselves? Where does this circle end, if there is a circle at all? Is it permissible by the catholic stance to defend a complete stranger? Can you enlighten me about this?
If I understand Catholic teaching correctly, then there is no circle, and yes, it is permissible to use force to defend yourself or anybody else from unjust aggression. The use of force in itself is not immoral, as much as the sport of boxing is not immoral. Should, in the situation of self-defence or the defence of others, death result from the use of that force, it is one of its two effects, the bad effect being the taking of the life of the unjust aggressor, and the good effect being the preservation of the life of the innocent victim.
Nevertheless, I will try to answer. Let me make sure I do understand properly. There are 7 people taken hostages, myself included. The terrorist offers me two options: 1) if I kill one of the other hostages, then all the others will be set free… and 2) if I decline it, then all the others will be killed, muself included. Is that correct? Or is it still imprecise? Maybe I will be let go free even if I decline? The difference seems to be important. If I will also be killed, then the “self-defence” applies. If I will be set free no matter how I choose, then I can use the following principle: "if the circumstances force me to choose between a “bad” and a “worse” options, I would choose the one which is “less bad”. In my eyes to save 5 people is less bad than allowing all the others to die. (By the way, that is not really a “terrorist scenario”, since the “terrorist” does not gain anything in either case. I would consider it a “crazy person scenario”.)
I actually wasn’t that specific in my own mind about it, and you point out several interesting distinctions.
If you were yourself to be killed, I still think it would be impermissible, as the innocent person is not the one who is threatening your life, and so it wouldn’t be self-defence. If you were to attack the hostage-taker to prevent him from taking your life and the lives of others, then that would be self-defence and the defence of others.
The distinction between Catholic morality and what you suggest is that you’re deciding on the permissibility of the action based on the consequences, whereas Catholic morality looks at the action itself to decide if it’s permissible.
So the consequence of the direct killing of an innocent person is the preservation of the lives of 5 (or 6) others, which from your perspective, makes the action less bad than not killing the innocent person.
From the Catholic perspective, the killing of the innocent person is objectively wrong under all circumstances and the consequences are irrelevant. The action doesn’t become less bad if it was a means to save 5 innocent lives. You’ve still killed an innocent person and killing an innocent person can never be right or “less bad” than not killing an innocent person. Your culpability in terms of sin may be reduced by your intentions or the fact you’ve acted under duress but what you’ve done is still gravely wrong because the actual act is the same as if you had killed the innocent person for any other reason.
Why is it impermissible? We always use “others” just like others are always “use” us. Is it impermissible because the “used” person will probably die? Would it be permissible to “use” this person by forcing him to throw big weight on the track to do the slow-down?
It is your very act of killing that person that brings about the good result. It is the direct consequence of his death. It is his death that slows the tram. In the scenario in the original post, it was not the death of that one person that saved the other five. It was merely a side effect of diverting the street-car.
Naturally. That would fall under self-sacrifice or suicide. Funny thing is, if the person would consider it self-sacrifice, the catholic morality would support it. If he simply wanted to end his life, it would be considered “sinful”. So complicated.

In my world it is his business and that is it.
I think so too, but I’m not entirely sure.
Yes it would. Obviously. This is a modified assisted suicide scenario with an added benevolent twist to it.
That was precisely my point and assisted suicide is exactly what I had in mind, but indeed with the added benevolent twist, so we could call it “assisted self-sacrifice.” I’m inclined to agree with your conclusion too, but I’d be interested to see how that would work out in Catholic philosophy.