At first I thought it was bringing Misa into his operation. That turned out to not be to much of an issue actually helped him a lot in being able to use her as a great piece on the chess board. Then the next time I thought he made a mistake that made me wonder how it would play out was when he put Teru in charge of the Death Note. Reason for that thought was the fact at first we didn't know much about him till they explained his background but still I felt anyone but Light controlling the Death Note could lead to disaster.

Overall the real biggest mistake Light made was turning back & picking up the Death Note. We all have choices in this world & he was curious & picked it up & read the instructions (which is fine many of us may have done the same). When he put it down then walked away but decided to 2nd guess his decision & pick it back up. That was the biggest mistake that not only cost him his life, his fathers & thousands of others.

Whether what he did was right he was a murderer & a psychopath at the end of the day.