Ever since Light Yagami stumbled onto a little black book that could kill with a simple name, his life took a dramatic turn towards murder. Starting from a relatively good-natured if not vain schoolboy and ending up a mass-murdering psychopath, Light Yagami used the Death Note's power to dole out his brand of unfeeling and self-righteous justice. Killing whoever he deems unworthy of the new world he wanted to create.
Though, despite having ultimate power, Light Yagami still needed to follow a set of rules to reach the end result of his god complex. Whether they be self-inflicted, a product of the Death Note, or necessary to his goal, they all played a part in his rise to power. Leaving the world as a game piece while he played "god."
10 If Light Gives Away Or Loses The Death Note
The Death Note seems to come with its special system to prevent anyone other than its owners from finding out it exists. If someone in possession of the Death Note loses it or gives it away, they lose all memories associated with it. A fact even Light Yagami cannot avoid, so instead, he put it to use.
Backed into a corner by L, Light used this particular rule to clear suspicion from him and his coconspirator Misa. The mind-wipe making them both truly believe they weren't Kira, preventing both from confessing after days of confinement.
9 Death Must Make Sense
All death caused by the Death Note must be logical. First and foremost, they cannot die by spectacular means. The book itself not able to make things like magic, dragons, or other fantastical objects appear. Secondly, it must be physically possible. If a disease is chosen to kill someone by a certain date, they will be given a disease that makes sense given the allotted time. If they die by an accident, it must be possible to occur within the time specified. If this criterion isn't met, they simply die by a heart attack.
Of course, this rule leaves much up for interpretation and that's exactly why Light experimented with it early in the series. Eventually finding the sweet point to make it an effective tool for him.
8 Age Limits
Despite its seemingly potent power over human life, the Death Note still has rules regarding the age of its victims. It refuses to kill anyone younger than 780 days old, leaving infants and newborns safe from the book's reach. On the flip-side, anyone 124 years or older all escapes the almost certain death the Death Note promises.
Thankfully for Light, neither of these age groups poses a problem to him. In a semi-realistic word like Death Note, they are both not physically able to try and solve the Kira case. In the case of people over 124 years old, they probably don't exist either. Leaving only those that can be affected by the note to try and stop him.
7 Spell Check
One particular rule that may have caused trouble for Light goes like this, "If a Death Note owner accidentally misspells a name four times, that person will be free from being killed by the Death Note." Meaning that Light has to be certain on how to spell someone's name before trying to kill them with the Death Note, otherwise, he'd effectively turn them unkillable by the Death Note by the fourth misspell.
The rest of the rule is even worst for Light, "However, if they intentionally misspell the name four times, the Death Note owner will die." Which means he can't willfully incorporate the first rule to his benefit.
6 About Other Death Note Users
Unlike everyone else, Death Note users have a particular skill that prevents their lifespan from being displayed to users who traded in half their life for Shinigami Eyes. While it may seem like a relatively useless gift, Light Yagami uses it to great effect in the series.
Having given up his ownership by relinquishing the Death Note to his father who traded in half his life for the Shinigami eyes, he is once again able to avoid suspicion when Light's Father, Soichiro, noticed that he could see his son's lifespan on his death bed. Leaving Soichiro relieved in believing that his son isn't Kira and giving Light ownership of the Death Note once he passed away.
5 Only 6 Death Notes
According to the rules of the Death Note, only 6 are allowed in the human world at a time. If a seventh were to appear, it would be rendered useless until one is either taken back by its Shinigami or it is destroyed. While one Death Note is dangerous enough, Light becomes a true schemer when he gets a hold of more than one.
Many of his plans included lending out different Death Notes to people, and shuffling them around for his personal use. To the point where even the Death Note the Japanese Task Force is used to his benefit by writing fake rules inside.
4 Cannot Extend His Own Life
The Death Note can have complete control over human death, but life is another matter entirely. No matter what it may say in the Death Note, life cannot be extended past its deadline. If the written time of death is after the end of a person's natural lifespan, they won't be given extra time and instead will just die naturally as they were intended.
Of course, someone like Light would immediately think to try and use the Death Note in such a way. Perhaps securing himself a near eternal life to look over the world he created. Fortunately, this rule stops that lofty ambition right in its tracks.
3 Can Never Take Back
Names written in the Death Note are fated to die. If it is erased, the person will still die. Even if the book is destroyed before they are killed, the grim hand of death will still strike, killing the person written in the book. No matter what happens, Death is permanent in Death Note.
Which is an interesting departure from the manga pilot, where it allowed the use of a Death Eraser that could bring people back to life. Then again, the series wouldn't be nearly as suspenseful if death was not a one-way street.
2 Never Fall In Love
Murderous psychopath he may be, but you have to admit that Light Yagami always kept his eyes on the prize. Forgoing many personal sacrifices to achieve his self-righteous goals, including never truly allowing himself to fall in love.
Try as she might, Misa Amane was never anything more than a tool to be used by Light. The young man using her utter devotion to manipulate her for his grand plan. As a Death Note user, she could kill others when he couldn't, and as his supposed girlfriend, he could maintain an appearance of normalcy to those that were suspicious of him. This also goes for every other romantic interest he had. He only used them to further his goal and was more than willing to get rid of them once they served their purpose.
1 Everybody Is Expendable
Light's romantic interests weren't the only ones that were expendable, everyone fit that bill. Innocent people were put to death if they ever threatened Light's goal, and even friends and family could be sacrifices if it meant their death was useful to him.
His father an unfortunate sacrifice in his attempt to kill off Mello. The young man begged his father to write Mello's real name in the Death Note before he passed away, all so he could get rid of another obstacle in his path. Though, while he may have felt something watching his father die, it doesn't refute the fact that he used his father's death to further his own goals.