I don’t think that the ending is so dumb and I think that [SPOILER]killing L is necessary. Killing L is meant to do 1 thing: it is meant to show that Light is a fucking bastard. The show progresses so that you care about both characters and it gives you hope that Light might not kill L because of the bond that seems to develop. But no, what ends up happening is that Light succumbs to killing L to win. It is symbolic of Light’s selfishness and meant to abolish any desire you might have to see him win. He is not supposed to be seen as a good guy. As soon as he kills L, he does more and more things like this further reinforcing this point.
Also, I think that it is important to see what happens after Light wins because we see that his utopia doesn’t come true. The future becomes fucked up and instead of having a perfect society, its severely flawed. This further invalidates Light’s purpose.
Finally, the ending is appropriate because it shows that humans are imperfect and that as soon as Light started involving more and more people into his plot, the more he set himself up to failure. Near got lucky, yes, but Near also took into account that Light’s tactic set himself up for a mistake to happen, which Near and Mello set up with Mello sacrificing himself. It also asserts that L was superior to M and N. What M and N had was Light making a mistake. L had Light in his sight forever, he just couldn’t do anything about it and as he drew close, he just got unlucky. You can’t blame him for getting himself killed by the shinigami. I really like how Light walked to his death and Ruykh took him down. I felt that the way that was presented was nice from an artistic point of view.[/SPOILER]
