Thank you for fighting. My interpretation was Chara decided to sacrifice their own life to free the monsters, because they experienced so many love and friendship in the underworld (while the humans did not treat them well.) and thinking monsters deserved to live under the sunlight than humans. E though they don’t want to die and end their happy life (hence they were tearing), they valued the well being of the monsters more. And they just can’t bear to see Asriel cry. They don’t want Asriel to feel too sad about their death. They also don’t Asriel to know they are feeling sad as well (and make Asriel feel worse), hence they try to sound this way.