The Gemara itself gives two reasons for the severity of this sin: 1. Because it is compared to murder, and 2. Because it is akin to idolatry, both of which carry the death-penalty.

Let us see why that is.

(a) It is compared to murder because seed is a potential human-being, so wasting it is as if one was callously destroying the baby that might otherwise have been born from it. Indeed, the very term 'waste' implies this. It goes without saying that the more useful and precious an object that one destroys, the greater the damage.

(b) One may perhaps add that Hash-m created man's seed first and foremost to form a baby, thereby making him a partner with Himself in the Creation. So besides the wasted seed, it is also a matter of a meaningful opportunity squandered.

(c) As for the comparison to idolatry, the Maharsha links this to the Gemara a little later, which describes this sin as 'inciting the Yetzer-ha'Ra (the Evil Inclination) on oneself', to the point that it dominates one (more than any other sin does). Once that happens, it gains power over the person, leading him from one sin to another, until it even convinces him to commit the sin of idolatry.

(d) The Gemara also writes that anyone who is guilty of this kind of behavior is not permitted to enter the vicinity of the Shechinah (Hash-m's Divine Presence). The Torah Temimah interprets this to mean that since his mind is constantly filled with lewd and immoral thoughts, it is impossible for him to fill it with pure and holy ones (and how can one stand before the Shechinah when one's mind [the most spiritual part of oneself] is full of negative thoughts).

(e) It also goes without saying that there is no room in his mind for Torah thoughts, and not studying Torah when one ought to, is a sin in itself.

(f) And, going one stage further, a person who behaves in this way relegates his status from a human being, who has been created in the image of Hash-m, to that of an animal, which, devoid of spirituality, lives only to fulfill its own desires. It is only natural therefore, that someone who divests himself of G-dliness is banished from the vicinity of the Shechinah.

(g) One final point; all of the above are caused by a sin which brings with it ecstatic pleasure, and the more pleasure one derives from a sin, the greater the sin is. The classical proof for this is from someone who eats Chalavim (forbidden fat) or who commits incest (both involving physical pleasure) b'Misasek (without any intention to sin). Chazal obligate him to bring a sin-offering, even though regarding any other sin (not involving physical pleasure) he would have been exempt.

I would say that this pleasure is the ingredient that magnifies the sin, as serious as it is to begin with, many times over, such that it caused not only the death of Er and Onan (Yehudah's sons), but the extermination of the entire generation of the Flood (as Rashi explains in the Gemara).

Be'Virchas Kol Tuv

Eliezer Chrysler