Death Note's Light Yagami is one of the most notorious anti-heroes/villain protagonists in all of anime and manga, but characters in Death Note aren't just simply "good" or "evil." Even during his descent into tyranny as Kira, Light shows his good side a few times, and not just when he's trying to butter someone up. In the following five instances, Light really did place the needs of others above his own.
When He Sincerely Wanted To Make A Safe, Just World For The Sake Of Others
Early on, Light Yagami saw himself not as a new God, but as a protector who needed to take justice into his own hands. Yes, he said "I'll become a god!" early in the series, but he was referring to the Death Note's immense power, not his role as a figure of worship. At first, Light didn't want legions of cultists or shrines built in his honor -- he just wanted to get the job done and make the world free from violent crime. Even if his methods were wrong, he at least had some positive intentions. Tragically, Light's ego would take over, and his extreme arrogance would alienate friends and foes alike.
When Light Scolded L For Nearly Upsetting Soichiro In The Hospital
Partway through the series, Light's father Soichiro Yagami suffered a breaking point. His stress levels had been climbing higher and higher with so much at stake, and one day, he collapsed from a stress-related heart attack and ended up in the hospital. Light and L both visited him, and L took that chance to show off Light's deductive skills by asking Light what sort of person Kira might be. Light gave his answer, and L used that data to suggest that Sayu Yagami, Light's little sister, might be Kira. Light erupted at once, alarmed that L's idea would alarm Soichiro and upset him during this delicate time. Fortunately, Soichiro wasn't actually bothered, since he was positive that his daughter was innocent, and he urged Light and L to settle down.
When Light (Without His Memories) Warned Misa Against Taking Risks
Misa Amane joined Light as the second Kira, but she soon got herself captured, and Light and Misa both lost their memories of the Death Note to prove their innocence (to make a long story short). Now they were under surveillance but otherwise fairly free, and Misa wanted to take some risks and try to find the third Kira (Kyosuke Higuchi). Light was aghast and urged Misa to not put herself in danger. Light also objected to the idea of L using Misa to investigate the new Kira, and he couldn't stand the idea of taking advantage of Misa's love for him. When Light's notebook memories were gone, the best side of him certainly shined through.
When Light Didn't Want Soichiro To Have The Shinigami Eyes
In a bizarre twist of fate, Soichiro Yagami ended up making the eye trade with Ryuk, gaining the power of a shinigami at the cost of half of his remaining life. All this was for the sake of raiding Mello's hideout and reclaiming the notebook, but Light didn't like this idea very much. Even Light felt bad about everything that Soichiro had been through, and he wanted someone else to have the eyes instead, such as Shuichi Aizawa or Hideki Ide. But Soichiro insisted, partly to soothe his guilty conscience about trading the notebook to Mello for Sayu's life, and Light had to stand back and let it happen.
When Light Spared Sayu's Life
Five years after L's demise, the hotheaded Mello decided to take matters into his own hands, ironically becoming a brutal criminal in the effort to take down the mass-murdering Kira. He tested the waters by kidnapping NPA director Takimura before moving on to Sayu Yagami. He'd only let her go in exchange for the notebook, and Light and Soichiro were agonizing over this dilemma. They'd be giving the mafia a massive weapon, but in good conscience, Sayu's father and brother couldn't just let her die.
In the end, Light and Soichiro both gave in and traded the notebook. For personal and practical reasons alike, Light rejected the idea of killing Sayu to protect the notebook. Mello won that exchange, but Sayu was safe, and Light would loosely cooperate with Near, another anti-Kira investigator, to make things right. Even Ryuk was pleasantly surprised by Light's course of action.