The grim tale of Death Note is a fascinating but eerie character study of Light Yagami, the brilliant but frustrated high school student who obtained the killer Death Note and used it to purge the world of crime. It can easily be said that Light slowly descended into villainy over time, but he wasn't the only one. The world's greatest detective, L, showed his villainous side too.
Both Light and L declared that they alone were justice and the other was a grave threat to the world. Light exploited and abused many people around him, including his second-in-command Misa Amane, but L isn't so innocent himself. Several times, L showed just how ruthless and callous he could be in his quest to triumph over Kira.
L Had a Criminal Die in His Place
L needed to test a few basic things about Kira before launching his investigation, so he forced a criminal to take his place on live television and present his real name: Lind L. Tailor. As far as L was concerned, it was all but guaranteed that this criminal would die, and he was willing to risk it. Sure enough, Mr. Tailor was killed at Light's hands when his fake-L broadcast was aired in Japan's Kanto region. L let a man die just to test Kira's nature, and oddly enough, Soichiro Yagami didn't object to this. Soichiro would later object to any attempts to use or kill criminals like that, but the first instance resulted in L one-upping Light at the cost of someone's life.
L Kept Misa & Light In Confinement Out of Sheer Stubbornness
At one point, Misa was captured as the second Kira, and Rem demanded Light find a solution to this. Light handed himself over to L to "prove" his and Misa's innocence, and sure enough, Light gave Ryuk a discrete message and relinquished the notebook, which Misa had also done. Without the notebook, they lost their memories of it and their powers, so Light and Misa were terrified and confused about what was going on while apprehended. L realized this, but he hated to admit defeat, so he kept them confined, trying in vain to squeeze out new information and resolve the Kira case his way. Shuichi Aizawa, Touta Matsuda and the other police objected because of the unnecessary cruelty, and L only let them go after performing one last, sadistic test.
L Kicked Light Twice
Light and Misa won conditional freedom, so Light was handcuffed to L while he and Misa were being monitored nonstop in his new headquarters. L then tested Light by pretending to give up hope on Kira and quit the case, prompting Light to slug him and demand that he continue. However, Light wasn't really picking a fight; he merely punched L to make a point, and L went too far when he kicked Light in the head, probably out of spite. This led Light to punch him back, and L kicked him once again. L was really the one who picked this fight with the supposedly innocent Light, and they only stopped when Matsuda called them on the phone to break it up with some interesting news about Misa's popularity.
L Worked With Criminals To Hunt the Third Kira
L was running out of manpower to hunt the new Kira, Kyosuke Higuchi, so he turned to seasoned criminals for help, which put a bad taste in Soichiro Yagami's mouth. In particular, L hired an expert con man, Aiber, and a suave burglar, Wedy. Not only was L working with seasoned crooks to go after Kira, but L's method of controlling them felt villainous, too. L didn't hire them as mercenaries; instead L had enough dirt on both of them to keep them in prison forever. L allowed Aiber and Wedy to operate freely as his minions, threatening life in prison if they defied him or ran away, proving how villainous L was toward his supposed allies.
L Wanted To Test the Notebook
Eventually, L managed to catch Kyosuke Higuchi as the third Kira, with some assistance from Aiber, Wedy, Light and the police. Now, he had the notebook, and L even saw and spoke to Rem, but that wasn't enough for him. It was clear that Higuchi had been acting as Kira and the notebook's power was real, but L still desired more evidence, and he'd buy it with blood. L wanted to have a criminal use the notebook to see it in action, and every police officer objected, since such a test was cruel and unnecessary. L even asked Watari, his handler, to ask every world government to find one that would gift L with a criminal for such test, again putting someone's life on the line to prove his point.