If you are talking about the Japanese comic "Detective Conan", that is: If I can guarantee your destruction, then, for the benefit of society, I will be willing to die with you. (From "The Last Case")

Actually, I like this sentence better: Put aside all the impossibilities, and the rest, no matter how incredible, is the fact. ("A Study in Scarlet")

There are many more, such as: Don't let a person's appearance affect your judgment. This is the most important thing. Emotions affect reason. ("The Sign of Four")

This great love: If I can get rid of the scourge of Moriarty for society, then I am willing to end my detective career. I can say that I have not wasted my life at all. If the journey of my life ends tonight, I can regard death as home with a clear conscience. The air in London is freshened up because of my presence. In the more than a thousand cases I have handled, I believe that I have never used my strength in the wrong place. ("The Last Case")

I don’t know which sentence you are looking for? Any tips?