What is Heidegger's main point of existentialism?

Heidegger's existentialism's main point is free choice.

Heidegger pointed out that man's freedom is absolute, because man lives in an isolated world, man is "thrown" into the world, and God, science, reason, morality, etc. are irrelevant to man, that is to say, they can't tell us the truth of life, the way of life, and at the same time, they have no influence on man. In other words, they cannot tell us the truth of life, the way of life, and at the same time, they do not have any controlling or restraining effect on human beings. Because of this, man has absolute freedom.

Existentialism recognizes that man's freedom is expressed in both choice and action. It is only through the actions that one chooses that one can recognize freedom, for the nature of man is determined by the actions that he chooses.

The freedom of the individual is manifested first of all in his realization of the lack of freedom due to his being bound by traditional culture and customs, and that therefore it is of the utmost importance for man to recognize the importance of choice and to act and take responsibility for his life in accordance with his choice.

Existentialist Theory of Tragedy:

For the existentialist, man lives in a world of opposition and disappointment to himself, and man's place in the world is uncertain. Absolutely free men are also troubled and unsupported loners.

Though man has freedom of choice, he faces a future life that is chaotic and purposeless. He simply moves blindly into the future, knowing only that the true end of life is death. Death, as the final destination of life, is of great significance to the existence of the individual.