The torah says that our lifes are the most important thing and we are permited to break virtualy every law if it will save our lifes. If you are offered to give your life to save many other lifes who would take presidence? EXAMPLE - If Hitler had come along and said to a jew if you commit suicide then I will not not carry out the holocaust. If you wont let me kill you then you are free to go home to your family but I will kill another 6 million jews. Who’s life is more important the life of the 1 individual or 6 million you cant have both. If he chose to kill himself would he be commiting a sin or doing a mitzva?
Let me refer first to your example. The Jew who was approached by Hitler (may his memory be obliterated) should save his own life. A mass murderer, a terrorist, or a vicious dictator is- not surprisingly- also a liar. Hitler would have the Jew commit suicide and then go on to kill everyone else. The reason why it is important to state the obvious (you were after all just bringing a theoretical example) is that Israel has believed the promises of Arafat and may now be entering a new "peace process" with Abu Mazen. So the premise of your example, while absurd, has been accepted by political leaders as acceptable. Let us remind ourselves: Someone who would murder another will not shrink from telling a lie. Regarding the theoretical side of the question: it is not permitted to take the lives of a few, or even of one, or even your own life to save the lives of the many except, according to some opinions, in extremely unusual situations- for example where the lives of those few intrinsically endanger all- both themselves and others. Another example, according to some halachic opinions is when the action taken is one that saves lives and only incidentally costs the life of another- a grenade is thrown at the window of a crowded bus and one of the passengers closes the window so that the grenade is deflected killing a single passerby.