Light and L were alike in many similiar ways, and prehaps the only difference between the two of them were the choices they made. Each one was living his own life by his own sense of justice and morality, and L had no problem using people as tools as well. On multiple accounts L used sentenced death penality criminals to confirm or deny his theories. Even if they were criminals, and they were scheduled to die that day, they were still people and you are still disrespecting the act of living in my eyes. L just drew the line farther back then Light.
I wonder if L might of used the deathnote in the same way Light had used it if he had found it first? Ultimately though, I think the basic message of this program is that there is only action and our actions and choices are what define us and our sense of morality, right and wrong, good and evil. Which in actuality, disproves the whole myth of good and evil.
Light was just a man trying to change the world, and his only mistake was that he became drunk with power. He made decisions that directly conflicted with his idea of changing and creating a perfect world. If I were Light I would of never of killed L. Lind Taylor, that was poor judgement on his part. I would of just gone about killing criminals through out the world with heart attack in silence and make the world a better place without using the deathnote to benefit myself in the process.
Light was right about one thing, No one could of done what he did, someone had to eliminate the vermin from this world, but he failed to realise that even he had failed to achieve that goal. He put himself before his intended goal which was originally to use the deathnote for good, and later the deathnote became a tool to turn himself into a god.