The ability to act aggressively and with determination is necessary, not just for survival, but also for success in one's life aims. Mars symbolises the impulse to act in this way, but left to itself can wreak havoc. The pursuit of one's goals in life is mainly represented by the Sun, which is generally more positive, both in determining those goals and in finding the appropriate path. This is because the Sun is more reflective, can see the bigger picture, realising that one's own goals impinge on other people, and that success in one's own mission may be diminished if others are trampled upon on the way.
The assertion of Mars is needed, but can be very damaging as it has none of the reflective capacity of the Sun. Mars does not consider the needs of other people on the way. The aims of the Sun cannot be achieved by Mars alone, because there may be nobody left to benefit from the exercise of those aims - they have all be vanquished. Well, they may have been vanquished but for suicide bombers - a subtle kind of warfare Mars finds it very hard to handle!
Herakles, a mythical figure thought of as a hero by the Greeks, is a Mars-like figure. He blunders his way forward, accidentally killing people, or wounding friends, on the way. Mars in the horoscope can show where, in an unreflective moment, we can harm other people in the pursuit of our goals.
Another figure in Greek myth signifying Mars is Ares. Ares was created in rage by Hera parthenogenically after her husband Zeus had created the goddess Athena out of his head without a mother. Thus Ares has no father. He acted out of pure instinct without any spiritual or intellectual dimension, and was successful where blind bloodthirstiness and brutality were all that was required. Where intelligence and reflection were required, Athena was more successful.
Mars gets things done. He is not concerned to compare his path with those of others, nor necessarily even to dominate others, but just to get to his chosen destination. In order to relate he needs Venus (Aphrodite) to whom he is irresistibly drawn, and who can tame him through beauty and pleasure. A good relationship between Venus and Mars in the chart can show a good balance between assertion and compromise.