Sure a world free of criminals and natorious beings would be nice, even so i would have to side with L on this one.
In the end Light became what he despised the most, a criminal. He killed pointlessly without fear or regreat. He believed that a world without criminals was ideal. He wanted to rid the world of evil. In order for him to truely succeed with this goal he would have to kill the whole human race after all no ones perfect. Even if he did manage to create this ideal world would u truely want him, a murderer, to rule it. Realy, some world that would be.
I believe that L was right in trying to take out Light. The power which Light obtained would prove to be too much to handel for any one person. Life and death souldn't be decided by any mere mortal.
Like I said earlyer I would have to side with L on this one, remember justice should always prevail