Death Note is a legendary psychological crime thriller starring the ambitious antihero Light Yagami, who seeks to transform the world with the murderous power of the Death Note, a gift from the shinigami Ryuk. Light made many enemies along the way, and even his allies sometimes worked against him despite their intentions.
Amane Misa once threatened Light's criminal empire when she got captured, for example, and in part two, something similar happened with Mikami Teru, the fanatical attorney who embraced Kira as the one true God. Teru seemed like the perfect Kira heir at first, but his role in the story actually proved how flawed and fragile Kira's kingdom was under the surface.
The Rise & Fall Of Mikami Teru
Mikami Teru made his debut five years after the death of L, when Light was waging a battle of wits against both Near and Mello. At the time, Near was making significant progress in uncovering Light's identity, and he convinced the police officers on Light's side to start watching Light closely. Now Light/Kira couldn't operate openly as Kira, and neither could Misa, so Light sought a replacement. At a rally in a Kira church, Light spotted Mikami Teru, who was making an effort to stand out. Light investigated Teru and approved of him, then sent him a notebook. Now Teru was a proxy Kira, performing all of Light's killings for him, complete with the shinigami eyes.
Teru worked hard for Light's sake with mechanical precision, but then Near started investigating him too, so on Light's orders, Teru passed the notebook to Takada Kiyomi while using a proxy so Near would be fooled. The whole time, Teru followed every order without question, making a tireless effort to advance Kira's cause. Teru and Kiyomi were poised to become the new king and queen of Kira's empire, but then Teru got sloppy.
During an emergency, he acted against Light's wishes and tried to kill Kiyomi to protect the notebook, only for this to expose him to Near, who replaced the real notebook with a replica. That led to Light's and Teru's downfalls when Teru unwittingly took the replica with him to the warehouse and tried to kill Near with it. Light was outraged at Teru's clumsy move, blaming it on Teru's excess zeal. Teru, being the loyal dog he was, couldn't sit still even when ordered to, and that cost him everything. All this exposes how fragile the Kira kingdom really was.
When Light Built An Empire Based On Threats & Crime
Light always intended to change the world and convince humanity to accept his views as the new god of justice, and while Light made remarkable progress in five years' time, he couldn't escape his enemies, including Near, the SPK and the Japanese police, and that cost him dearly. Light's criminal career was based not on shonen-style cooperation and trust but on manipulation, deception and threats, and while such methods can be effective, they are not useful in the long term. Light kept twisting the world around him until it snapped and his house of cards came crashing down. Truth and respect always trump lies and manipulation, and that's why his kingdom fell.
No matter what Light did, he couldn't keep running from his enemies, who would never be swayed to his cause and join Kira's kingdom. Light increasingly relied on dirty tactics and manipulation to stay ahead, but he was acting on borrowed time and running out of people on whom he could truly trust and rely. That is the fate of most villains and criminals in fiction, whose misdeeds catch up to them sooner or later. Light was forced to rely on tricks and zealous followers like Teru to keep his empire going, but that was a sign he was getting desperate -- and sure enough, Teru became a massive liability in the end.
A noble, heroic protagonist can entrust their life to an ally, while a schemer like Light must turn to eccentric characters like Teru, and even if Teru was zealously devoted to Kira, villains like him can't be counted on in the long run. Teru's nomination as Light's replacement was an early warning sign that the Kira kingdom was on the verge of collapse, and sure enough, it all fell apart sooner rather than later.
