Death Note is widely considered to be one of the best anime series of all time and features a fantastic story complete with an excellent cast of complex and compelling characters. L and Near in particular provide plenty of intrigue as well as some truly exceptional moments, but it's perhaps the series' main protagonist, Light Yagami, who brings the most to the table in this regard.
Light starts out as a regular, albeit incredibly intelligent, high school student, but things take a turn when he finds a notebook with the power to kill those whose names are written inside. He decides to use the death note to rid the world of evil by killing its criminals, but soon draws the attention of authorities worldwide. He very nearly gets away with his crimes, but his efforts to evade detection are constantly hampered by his own foolish mistakes.
10 Experimenting With The Death Note
While it makes sense that Light would want to learn the limitations of the death note, the way in which he tests them out forces him to give up the element of surprise. He effectively lets L know that he's able to control a person in the moments leading up to their death, which could have played a vital role in taking down his bitter rival.
Light's coded messages were also a mistake, as they provided L with hard evidence that Kira actually exists and hinted at the possibility of supernatural forces being involved. L then confronts Light with this evidence while assessing his suitability for the task force, and Light's reasoning that the words must be out of order rather than there being some missing only makes L suspect him more.
9 Sending Demegawa After The SPK
Near had plenty of reasons to suspect that the new L was Kira, but very little hard proof. If there was even the slightest bit of doubt in his mind, however, it will have been all but washed away when Light decided to send Demegawa and an angry mob to hunt down the headquarters of the SPK almost immediately after being confronted.
Granted, were it not for Near dumping millions of dollars down onto the streets below, Light's plan would probably have worked. Through its failure though, Light not only confirmed his guilt to Near, but also to members of his own team. His decision to then allow Aizawa to contact Near rather than just killing the task force off was also foolish and played a big part in his eventual downfall.
8 Trusting His Father To Kill Mello
Soichiro Yagami regularly refused to even carry a gun, let alone use one. With this in mind, it was foolish of Light to expect his father to use the death note to kill Mello, as even a blind man could have seen that he was never going to actually go through with it. In Light's defense, the manga makes it quite clear that Yagami senior put himself forward for the role, but Light could have still made the plan work with a few extra steps.
Had Light simply got his father to wear a body camera during the failed mission, he would have had an image of Mello and could have either killed him himself or had Misa take care of it a little later on. Instead, his father dies for nothing and Mello lives on to fight another day; eventually playing a pivotal role in Light's defeat.
7 Becoming The New L
To a certain extent, it's understandable why Light would want to retain a position on the task force, as his continued involvement allowed him to not only keep up to date with the investigation, but also to sabotage it should the need ever arise. That said, L's death provided him with the perfect opportunity to step back from the front line, and it's one that he really should have taken.
With his name already cleared and both Watari and L dead, Light had absolutely nothing to fear from the Japanese task force. By assuming L's identity, however, he effectively makes himself the prime suspect almost as soon as Near and Mello enter the picture, which would not have been the case had he left the task force or remained a regular member of it.
6 Killing Lind L. Tailor
Not only was killing Lind L. Tailor incredibly foolish, but it was also a little out of character on Light's part. His actions are usually fairly calm and calculated, but, in this instance, he rises to L's bait and his impulsiveness allows the detective to narrow down Kira's location to the Kanto region.
Had he taken just a moment to think about it, Light would have realized that the world's greatest detective would never be so foolish as to show both his face and name on live television. Instead, he jumps at the opportunity to seemingly dispose of his pursuer, and, in doing so, gives away both his location and, to some extent, confirms the existence of Kira.
5 Killing Raye Penber
Though brilliantly conceived and almost perfectly executed, Light's plan to kill Raye Penber was entirely unnecessary. The FBI agent was convinced that there was no way that Light could possibly be Kira, but, by killing him, Light once again throws himself squarely into the spotlight.
At that point in time, very little was known about Kira, nor was anybody aware of the existence of a second Death Note. With that in mind, Light would have been much better served having one of the other people that were being investigated write a note confessing to being Kira before their death and then laying low for a while until the investigation was over.
4 Underestimating Near
Granted, Near got incredibly lucky at numerous points throughout Death Note's final cour, but he's still an excellent detective. This is perhaps unsurprising really, considering that he was trained to one day become L's successor. He quickly identifies Light as being Kira and outsmarts him several times leading up to the final showdown.
With all this in mind, it's somewhat astounding that Light didn't have a plan B, especially given how easily Near could have tested the Death Note and figured out that it was a fake. This overconfidence was arguably the result of his overgrown ego and delusions of grandeur, but it was a fatal mistake nonetheless.
3 Failing To Use Halle Lidner As Part Of His Plan
Light quickly identifies Halle Lidner as being an SPK plant and yet, for whatever reason, opts to do nothing with this information. Having only just used the death note to learn the location of Mello's secret hideout and the identity of several of his accomplishes, it's pretty surprising that Light doesn't take a similar approach with Halle.
While it's true that she's using an alias, her proximity to Kiyomi Takada would have made it very easy for Teru Mikami to learn her real name. From here, Light could have used the death note to have Halle provide him with more information about the SPK, or even have her wear a secret camera to the organization's HQ to get images of Near and the other SPK members.
2 Placing His Life In Teru Mikami's Hands
Given everything that Light went through with Misa, it's a little baffling that he would trust somebody that he had never met to carry out his bidding. To be fair, Teru Mikami does, in many ways, understand Light's motivations and way of thinking, but placing so much faith in Mikami's compliance is ultimately what costs Light his victory.
By the end of the series, Light sees himself as a God and doesn't even consider the possibility that near might win. However, for this blind faith to extend to Mikami - a man that he has never actually met - does not make sense. Light should have had a plan B, even if it had been something as simple as having Mikami tear out an extra page of the Death Note before hiding it just in case he ever gave away its location.
1 Picking Up The Death Note
Light's initial reaction to finding the death note is to dismiss it and leave it on the ground. Unfortunately, however, curiosity gets the better of him and he soon returns to pick it up; setting into motion a chain of events that would ultimately lead to his death. Had he only kept on walking, he could have gone on to live an exceptional life.
Readers and viewers don't really get to know Light all that well before he actually finds the death note, but they do get a pretty good idea of the kind of person that he is after he gives up ownership of it. He's shown to be an incredibly smart young man with a passion and aptitude for solving crime, which in turn demonstrates just how big of an effect the death note really has on him.