What is the Most Severe Transgression in the Torah?

Greetings, The most severe transgression in the Torah is murder. See Rambam (Laws of Murderers and the Preservation of Life, chapter 4, halachot 8-9) who writes that one who kills people — and the two witnesses did not see him at the same time, but one witnessed after the other, or he killed in front of two witnesses without warning, or the witnesses were disqualified through investigation but not examination — all of these cases involving murderers, they place them under a dome and feed them bitter bread and water until their bowels churn. Afterward, they feed them barley until the tumorous swelling bursts from the illness. This procedure is not done for other capital offenses punishable by the court, but only if one is liable to death. If he is not liable, he is exempt. Although there are grave sins, such as murder, they do not cause the destruction of the world’s habitation as murder does. Even idolatry—and certainly forbidden sexual relations or desecration of Shabbat—are not like murder. These sins are transgressions between man and Hashem, whereas murder is a transgression between a person and his fellow. Anyone who has committed this sin is a complete wicked person. All the mitzvot he performs during his life do not compare to this sin and will not save him from judgment, as it is written, “A man is ensnared by the blood of his soul...” (Proverbs 28:17). Consider Ahab, who worshiped idols, as it is stated, “Not so was Ahab” (I Kings 21:25). When his sins and merits were scrutinized before the God of spirits, no sin was found that obligated his destruction; there was only the sin of wine money, as it says, “Then the spirit went out and stood before Hashem” (ibid. 22:21; II Chronicles 18:20) — this is the spirit of wine money — and it was told, “You shall entice and also prevail” (I Kings 22:22; II Chronicles 18:21). He is the wicked person who did not kill by his own hand but incited it. How much more so one who kills by his own hand! Therefore. See also the words of Rabbi Gershon Zilberstein, shlita, in his book "Cheshukei Chemed" on Tractate Yoma (pages 363-364), which discuss a person who sees one pursuing to kill his fellow and another pursuing to assault an engaged woman, and can save only one of them. Perhaps one should save the one who is to be murdered in view of the aforementioned Rambam indicating that forbidden relations are considered as light sins compared to murder. 

Blessings, Hillel Meirs