Ok, so this is my guess for this overload of depression:



1st: Frisk killed Carrot (Swap Papy)



2nd: Blueberry found out



3rd: Blueberry then found Frisk, while holding Papy's hoodie



4th: Blueberry killed Frisk to avenge his older brother



5th: Blueberry either continuously said the words "I just want my brother back" and "I just want him back" while near the Echo Flowers, while, of course, wanting to undo or get another chance to prevent the death of his older brother, or he only said that once, and the Echo Flowers were just...doing what they were made for. Echoing his voice. (Since the Echo Flowers, as most of us know, were made so Asgore wouldn't forget things...)



This is, of course, only my guess of what happened, therefore you can correct me or anything like that, but hey, I tried.

...I think I can feel tears in my eyes now hang on ;;~;;