Rem killed L in Death Note.
While L was closing in on Light Yagami's identity as Kira, Rem, a Shinigami (god of death), intervened to protect Misa Amane, who would have been caught and likely executed as an accomplice if L had succeeded. Rem wrote L's real name, L Lawliet, in her Death Note, sacrificing herself in the process. This act directly caused L's death, as dictated by the rules of the Death Note. Light orchestrated events to manipulate Rem into making this choice. Therefore, although Light didn't directly write L's name, he bears responsibility for L's death due to his manipulation of Rem.
