Against a ruach rah (evil spirit) David HaMelech sang: Lo iegurechah rah va’atsape, “Evil shall not dwell with You” (Tehillim 5:5). And regarding he who impurifies himself with the pleasure in the evil that profanes men the most, to wit, in the hotza'at zera levatalah (spilling the seeds in vain), it is thus said: “He will not come inside the masach [curtain] of the Holy One, blessed be He, and behold the presence of Atik Yomim” (Zohar I:188a, Vayeshev). Why it is that semen has such power to bring tumah? For the spiritual law of zeh l’umat zeh asah Elokim, “What G-d created in the side of kedusha He equally created in the side of tumah” (Kohelet 7:14) establishes that, semen is as much the rawest and most naked ‘creative energy’ that exists in the physical universe, the actual corporification of an aspect of the Ohr Ayin Sof through which even the neshamot of the tzaddikim are brought down to this world, as well as the potential source for the greatest spiritual degeneration that there is, implying on the direct strengthening of the sitra achra, chaz v’shalom. Hence, such a precious and powerful energy must be transported and used with the greatest care and discretion in order not to allow that, “They shall profane My secret place; for the robbers shall enter into it, and profane it” (Yecheskiel HaNavi 7:22). And the appropriate manner to maintain this energy hidden, and thus protected, is through its ‘exclusive’ use in the procreation of life – when the love of a man and his wife act as a spiritual barrier against the chitzonim – the insatiable “robbers” from the side of profanity.
Now, if the person transgresses the commandments of Lo Tirtzach, Lo Tinaf, Lo Tignov, Lo Ta’ane Bereacha Ed Shaker, “Do not kill; Do not commit adultury; Do not steal; and Do not bear false witness” (Shemot 20:13), and in this way becomes a “rebellious prince and companion of thieves... Therefore said the L-rd, the L-rd of hosts, the mighty One of Israel, Ah! I will ease me of My adversaries, and avenge me of My enemies” (Yeshayahu HaNavi 1:23-24). And this is why this perversity is so vile, for it implies on the transgression of so many Divine commandments: “Do not kill”, since it is said that “Your hands are full of blood” (ibid. Yeshayahu 15. The previous Zohar 188a explains that this verse alludes to zera levatalah. See also Rashi on Yecheskiel HaNavi 23:37; R’ Chayim Vital zt”l, Sha’arei Kedusha, Chelek Beit, Sha’ar Chet), and truly, each seed could have generated life, but it only result in death (thus, tumat hameit). “Do not commit adultery”, for instead of increasing the powers of Good in the world, this person giver power to the “lover”, i.e., all that is external to the kedusha, chaz v'shalom (see Kitsur Sefer Chareidim, Chai Adam, in the subject of not being Mnof by himself, by spilling seed). “Do not steal”, for the person becomes a direct accessory to stealing the Ohr Ayin Sof, causing tzaarot [sufferings] to the world. Finally, “Do not bear false witness”, for as it is written, “The L-rd your G-d will circumcise your heart, and the heart of your seed” (Devarim 30:6), which means that, whatever happens to man also happens to his offspring, be it for good or not. And “when a man dies, the progeny of mashchitim [spiritual destroyers] that he fathered [through the energy of his semen that fed the ‘spiritual lover’, the sitra achra] also follows after the bier [together with his earthly children], expressing great sorrow [and ‘bearing false witness’ to the ‘generosity’ of his father which, after all, begot them]. But all this grieving [and false witnessing] is a terrible disgrace for the deceased’” (R’ Nachman of Breslov zt”l, Likkutei Moharan I:141). May Hashem spare us from such punishments.