We humans exhibit a most amazing proficiency in the skill of rationalizing our acts, beliefs, desires, and so forth. We are so proficient at it, that even those who seem mentally inferior in the extreme can--when accused of wrongdoing--stun you when you witness how rapidly and easily they produce intricate and compelling excuses able to transform even the most heinous of deeds into acts of august merit. No matter how incapable of avoiding even the deepest depths of poverty and illiteracy, virtually every human being's ability to defend the integrity of his or her ego reaches the pinnacle of success very early in life. This ability never recedes from that pinnacle no matter how long life may last.
That capacity is particularly stunning when it comes to rationalizing direct assaults upon the life of what is known, or reasonably suspected to be, an innocent human being. Wherever abortion, euthanasia, and the deliberate slaughter of non-combatants in warfare promise a quick solution to pressing problems, most people can easily produce "iron-clad" arguments "proving" the seemingly murderous solution is actually "love", "the most moral thing ever done", "an ambition of the highest and noblest kind which ever perhaps existed", or some other such awesome thing. It's commonly called using the end to "justify" the means.
But, when you use an august end to justify the means, you attribute to the means a sublime utility, and, thereby, necessarily deify it. Therefore, when you say that directly killing the innocent can sometimes serve such exalted goals as "love", "the most moral thing every done", and "an ambition of the highest and noblest kind which ever perhaps existed", you attribute to murder a utility so exalted, you necessarily worship murder whether or not you intend to do so.
In the letters, which comprise this book, the author is repeatedly hammering away at this business of rationalizing abortion, euthanasia, and the targeting of non-combatants in warfare. He is repeatedly trying to show that to attribute sublime utility to these three forms of murder is a kind of mental illness, which must eventually have exceedingly tragic consequences for all who nurture that mental illness within themselves. Above all, he tries to warn the "murder worshippers" of the one eternal consequence of their action--namely this: In death's all-revealing light, they shall behold for a fact that their tactic was indeed--by God's eternal standards!--nothing more than a diabolical attempt to worship murder. Seeing that, they will not fail to hurl themselves into hell in an unbelievably fanatical and hate-filled attempt to curse God forever. And for what?! Because His Infinitely Informed Truth dares to defame that greatly what they spent their whole lives insisting was something of Sublime Utility!