Dr. Serizawa: The Oxygen Destroyer cannot be used.
Ogata: If we don't defend ourselves from Godzilla now, what will become of us?
Dr. Serizawa: And what will become of us if a weapon, such as I now have, falls into the wrong hands?
Ogata: Then you have a responsibility no man has ever faced. You have your fear which might become reality. And you have Godzilla, which is reality.
Dr. Yamane: They are so wrong. Godzilla should not be destroyed. He should be studied.
Security Officer: Did you hear that? Godzilla.
Security Officer: He claimed he saw a monster, a horrible monster.
Steve Martin: If he saw a monster, he's had too much saké.
Steve Martin: I'm afraid my Japanese is a little rusty.
Answer: I don't remember if that line was in the KOTM American cut as well, but in the Japanese Gojira cut, Serizawa, as he is talking about why the capabilities of microoxygen and the Oxygen Destroyer horrify him, mentions that he fears that "perhaps one day he might have to create the Oxygen Destroyer again," and that horrified him. The reason he allowed himself to die didn't have anything to do with stopping Godzilla; it was to ensure that no-one, including himself, would be able to create the Oxygen Destroyer ever again. That was his entire arc in the movie. He has previous scenes in the movie where he explains why his discovery of microoxygen and its destructive capabilities horrified him, and before he went to unleash the Oxygen Destroyer for its first and final time, he had ensured to destroy all the research in his lab that could be used to recreate the Oxygen Destroyer. Finally, he allowed himself to die so that he would erase the possibility of himself recreating that weapon as well.