הבל

הבל I think gets a really bad rap when you read קהלת. Usually translated as vanity or something empty. I think it's a mistake.

Consider how much importance הבל gets:
1) name of הבל בן אדם
2) our entire life being defined by 7 הבלים - see הבל הבלים אמר קהלת הבל הבלים הכל הבל
3) specifically הבל פיהם של תינוקות של בית רבן
4) kabbalistic writings

and yet then why is it seen so negatively as הבל ורעות רוח.

I'd suggest the pain of הבל is in encountering the wisp (or maybe better the halo as in אור מקיף) of true existential being and meaning. The wisp of it but not the source itself, hence the angst and frustration. Requires further explanation.

Also how is קן/קנין the death of הבל? How is שת the substitute of הבל?
שת לי אלהים זרע אחר וגו' all questions that need to be considered...