Death Note is a dramatic, serialized story following the machinations of Light Yagami (otherwise known as Kira) as he descends from a promising student to a mass murdering sociopath. Unlike other anime which orient around massive explosions, over the top battles, and dramatic monologues, the conflicts throughout the 37 episode series revolve around mental and psychological battles far more than physical ones.
Like every anime protagonist, the "fights" Light engages in produce varied results. For better or worse, each decision made fundamentally shifts the direction of the story, culminating in a cathartic, albeit poignant ending.
10 Smart Decision - Working With L
L Lawliet serves simultaneously as the series' antagonist and hero, proving a worthy intellectual rival to Light. Near the beginning of the series, Light comes under L's scrutiny so intensely that Lawliet reveals himself to the young student in person, professing his identity as the famous investigator.
After an interrogation, L extended Light an invitation on the Kira case. The latter gladly accepted the opportunity as it granted him access to the inner workings of his pursuers. Though it would give Lawliet a fuller insight into Kira's true identity, it would cost him his life well before his conviction.
9 Dumb Decision - Lashing Out
During their collaboration of the Kira case, L was content to observe Kira's actions before moving forward. Frustrated at his "partner's" patience and attempting to play the role of a passionate investigator, Light abruptly slugs Lawliet, sending him reeling.
Yagami was expecting both to pressure L into a poor choice and to make himself out as a vigilant investigator. However, given that L already suspected him, it did little to force his hand. Additionally, it was the only physical outburst Yagami would ever attempt, making his actions seem erratic and suspicious rather than excessively gallant.
8 Smart Decision - Recruiting Misa
Misa Amane owned a second death note separate from Light's, using it to kill carelessly in the name of her "righteous" benefactor. Though Yagami initially perceived her as an obstacle to be disposed of, he quickly realized how useful a loyal pawn could be.
Misa substituted for several roles Light was incapable (or unwilling) to fulfill. She halved her life span to gain the shinigami eyes (which would read the names of those she saw), slew when he was under too intense scrutiny to use the notebook himself, and would later be indirectly responsible for L's downfall.
7 Dumb Decision - Using His Father's Police Archives
As the son of the chief of police, Light Yagami found himself in a unique position to receive information about targets which would otherwise be unavailable to ordinary people.
However, it was because the information was so secretive that L could significantly narrow the scope of his search and increase the security he would place on Light's home. While Light gained viable intel through the database, it was foolish not to have considered its implications, especially since he already had a taste of L's competence.
6 Smart Decision - Blackmailing Penber
Unlike extracting information directly from his father's computer, Light's coercion of Raye Penber proved a risky though rewarding endeavor. Through well placed blackmail and coercion, he managed to extract valuable details about the American investigation that would have otherwise been impossible to know.
Light's meticulous, methodical extermination of the teams incurred America's withdrawal from the case. In the absence of a continued investigation, his wife, Naomi, would assume his mantle, only to meet a similar fate. In the end, it resulted as a clean victory in Light's favor, with none the wiser for it.
5 Dumb Decision - Giving The Notebook to the Yotsuba Group
The Yotsuba were a coalition of heartless, driven businessmen who were given the Death Note while Light was in solitary confinement. They were tasked with continuing Light's work in his absence to give the illusion that he was not the one responsible for the murders. In exchange, they were able to use the notebook to kill economic competitors.
However, the slight deviation in killings was not unnoticed by Lawliet, and he suspected it was related to Yagami's captivity. If Light had given it to one his fanatics (such as Teru Mikami), the deception would've implicated him less, not more.
4 Smart Decision - Volunteering For Confinement
When suspicions for Light were at their highest, he volunteered to put himself in solitary confinement, the only place more certain for him not to cause mischief than being physically attached to L.
While Lawliet was not convinced by Light's actions, it emancipated him in the eyes of the remaining task force. It resulted in Yagami genuinely losing his memories of the notebook; while a hindrance, it would make his acting more convincing and reposition himself to deliver the killing blow to L.
3 Dumb Decision - Attempting To Write Names When Exposed
Eventually, Light's luck and resources ran out, and he was exposed as the Kira murderer. Confronted by armed police, he backed into a corner, slowly becoming unraveled as his empire of lies imploded before his eyes.
He could have allowed himself to be arrested, struck up a deal with Ryuk for the shinigami eyes, and scribbled the names of his police escort in the transport as he was being carried off. Instead, he attempted to write in full view of armed and furious officers who had guns trained to his torso.
2 Smart Decision - Sacrificing Ren to Kill L
Over the time Misa had come to know her shinigami, Ren, they had formed a bond much closer than Yagami's with Ryuk. It did not go beneath Light's notice. He arranged events in a matter that would correctly implicate Misa as Kira before the police, should Lawliet continue to investigate.
Aware of Yagami's scheme though powerless to stop it, Ren defied the rules of the Death Note by writing L's name. Her actions assured their mutual demise. She cursed Yagami to her dying breath, but they both got what they wanted ~ L was dead, and Misa remained safe.
1 Dumb Decision - Falling For The Broadcast
One of Light's earliest mistakes was also one of his greatest. When L announced "himself" and before the world as the one who would catch Kira, he was more than eager to prove his strength by killing him instantly.
However, it was a ruse ~ the man on the screen was a convict who negotiated a reduced sentence for his service, and the broadcast (advertised globally), was only released in Japan. Thus, L could safely eliminate billions of candidates from his search, already hot on the heels of the Kira killer.