Context: In the 26th episode of Death Note, Light finally defeats L after regaining his memories of being Kira and making Rem write L's name into the death note for him. Right before L dies from his heart attack, Light privately smiles in his face, letting him know that he was truly right the entire time and Light was in fact Kira.

This probably wasn't THAT groundbreaking of a revelation for L, as he did heavily suspect Light and Misa of being the two Kiras before his death and was pretty much sold on Light being Kira as shown by his actions prior to this scene (asking Light if he's ever actually told the truth, washing his feet as symbolism of Jesus washing the feet of the traitor Judas, etc), he just lacked conclusive proof.

However, even for someone as smart as L, there will always be a slight doubt in the back of your mind, the possibility, however small, that you may be wrong. If I were to represent this with percentages, I'd say L was 99% sure that Light was Kira. But in cases as severe and complex as the Kira case, that 1% can mean a lot. It could be argued that Light revealing himself to L, though done primarily to gloat in L's face that he won, was in fact a partly good thing to do as L deserved to have some sense of closure by having his beliefs finally confirmed after the intense cat and mouse game that they had played. He deserved to have the possible 1% doubt in his mind satisfied.

Morality of letting the detective investigating you know that he was right all along before he dies?