Complex sentence: Many forms of Existentialist thinking essentially begin with the premise that life is objectively meaningless, and proceed to the question of why one should not just kill oneself; they then answer this question by suggesting that the individual has the power to give personal meaning to life.
Split into simpler sentences: Many forms of Existentialist thinking essentially begin with the premise that life is objectively meaningless, and proceed to the question of why one should "not just kill his or her self?". They then proceed to answer this question by suggesting that the individual has the power to give personal meaning to life.