Weeell~ both Light and Saguru are really intelligent 17 yr olds who like to solve crimes and stop criminals.
They'd probably have a lot to nitpick over on the How (as in, does the end justify the means?), and also the Why...
(Oh good grief, have just had the mental image of Hakuba asking Yagami WHY he kills all those murderers.)

-Read it in one of my japanese learning books (that are woefully discarded). Is the origin of Watashi I believe. ^_^;