Utilitarianism revolves around the idea that the worth of an action is the sum of the aggregate pleasure it creates for people. If an actions creates more good for society than it does bad, than it is a decent action. Under this line of thinking the means can justify the ends! During his insane soliloquy at the end of the series Light concedes that 'that killing people in itself is a crime', but that it had to be done.
For him, murdering people was a necessary crime to fix this 'rotten world'.
What strikes me about Light’s character is actually how perversely noble he is (that is not to say I agree with what he did). Of the five people who obtain and use the Death Note in the series: Higuchi, Light, Misa, Jack and Mikami (and possibly Near if Matsuda is to be believed) only Light and Mikami (under Light’s guide) used it for selfless purposes.
Higuchi used it to advance his career and Misa used the Death Note first to get close to Light, then later to impress him. While Jack Neylon utilized it for purposes of organised crime. Light on the other hand used the Death Note purely to improve the world; and never for personal gain.
It’s hard to overstate just how few people in the World would ever do this. The Death Note represents the perfect opportunity to get away with anything you want for no cost.
Getting back to Light’s particular philosophy, it’s interesting to note that is his conception of using the death penalty as a deterrent is actually extremely successful (its success in reality is dubious at best). Crime rates drop drastically, organised crime and all that entails (slavery and the drug and sex trade) have almost completely ceased to exist and war is a thing of the past. This clearly provides a very real good for society.
In Yagami’s mind, the pain felt by criminals is nothing compared to the new found safety of innocent men, women and children and therefore his otherwise cruel actions are excusable.
Yagami’s beliefs come in the context of the Japanese legal system. Prosecutors often won’t take on criminals who haven’t confessed or there isn’t overwhelming evidence against in order to maintain a 100% conviction rate. Because of this many criminals walk free in Japanese society. The Japanese films make this element even more explicit than the anime with some of Light Yagami’s fury being actively caused by this absence of justice for the victims after he hacked into police computers and was horrified by the number of people who simply walked free.
Personally, I can’t help but wonder if Light Yagami’s utilitarianism might actually have been prudent within the context of his Universe. One could argue that it was a society based around fear, not justice, and while criminals certainly have a lot to be afraid of, for the common working man or woman I don’t think this holds true. Light Yagami is brilliant, and is unlikely to make errors when it comes to killing criminals, and so won’t kill innocents by mistake. While it’s likely some innocent people who have been arrested for crimes they didn’t commit are killed erroneously, I don’t think this would have been common. Light Yagami being too self-righteous to kill people whose guilt is uncertain. Furthermore, most people (from what we see) actively support Kira which would seem to imply that very few innocent people have been murdered by Kira (with the obvious exceptions of Raye, Naomi and other police officers who attempted to apprehend him).
With war and crime being a thing of the past, it’s fair to argue less people die by Light’s metaphorical hand than would have from crime in normal circumstances.
In 2012, the second last year of Light Yagami’s reign in the anime (and the only year I could find data for) over 437,000 people (approximately) were murdered. In the universe of Death Note, Light says, “Global crime rates have been reduced by over 70%.” So lets apply this number to how many people were murdered under his rule that year, and 437,000 in that one year becomes 131,000; a difference of 306,000. This means 306,000 people were saved from murder in that one, hypothetical year. This number, if you assume it holds true to the four years Light rules after the death of L, becomes 1,024,000.
This naturally excludes lives indirectly saved such as those would no longer die from the drug trade in the absence of major cartels. It also doesn’t factor in the great reductions in other horrific crimes and the beneficial impact that will have on many innocent people’s lives. Light Yagami was in many ways therefore justified, though he killed many, less people died on the whole and so it can be successfully argued that the means justify the ends.
One thing is certain though, even if you don’t agree with what I’ve written, Light Yagami is definitely a supporter of capital punishment.