Since your Dandy's world slasher au is now greenlighted, and i love it and also, i have some questions:
2. What were the motivations for each of the killers to become what they are now? I think there were more motivations than the cancellation of the live action program in which they were child actors...
HIII I'm glad you love the au! It's the one I'm most excited to work on now and I have a lot prepared for it >:D
Now answering your questions:
1. Yes he is! But he is just a little orphan kid that has a connection to one of the killers... Which involves his dead parents.
2. Dandy killed Devan and since he got away with it once, now he kills anyone that slightly annoys him. Astro basically cleans Dandy's cases most of time, it's because of him that Dandy gets away most of time.
Astro slowly become obsessed with Cosmo because of how close to home Cosmo feels. Cosmo's family are immigrants, and kept their culture so lively and Astro sorta admires that for missing their homeland and their family that they never met...maybe it's just a mix of envy and at the same time admiration. The more Astro learned about Cosmo (by stalking) the more they got obsessed.
Sprout thanks to a traumatic household ended up affecting a lot how she saw relationships and people in general, at some point, she killed a guy she was dating when she was freshly in her 18, and she got an acquired taste on doing so. As she was struggling with food and finding a job she sort of just got creative with the way she could get rid of the body so... yeah! First time eating human flesh! After she actually started to living with Cosmo, she stopped killing people that often, and started to actually date someone she could feel actual love. However, she still likes getting a taste of human flesh once two months.
Shelly felt a strong sense of justice after Tisha got fired because of someone from her past that couldn't let go of her, so she killed that person... And alas, she also got a taste on doing justice with her own hands. But she knows it's wrong and if she gets caught she will just ruin Tisha's life again... And she wants her to be happy.
Vee always had this vengeful and bitter side to her, by ruining other kids jobs, trying to always get on top and seeing everything as a competition... stuff that she dealt with with therapy and her older sister Veronica in her life. However when Veronica died, part of Vee died and she ended up killing one of the persons she thought was an obstacle in her life... Then she started to make her murders a bit more fancy.
Dyle got quite obsessed while investigating some murder cases of an specific killer that isn't important at the current storyline, but spoiler: It's Bobette. He always was a fan of criminal cases, and had a morbid curiosity to it as much as his love for his other hobbies... killing turned into a hobbie for him, and actually got to perfect it once he and Vee joined forces on making actual great cases... They both sort of become perfect partners in crime, with one being part of the justice system and manipulating the cases, while Vee is in control of the local media.