I can only answer minor questions here... I haven't really watched or read most of these... [School hit me like a truck] I'm planning on reading tokyo ghoul and That time I got reincarnated as a slime :>
1. Boku no Hīrō Akademia = Why did Kaa-chan (Bakugou) bully Deku (Izuku) when they were younger? At what age did it start?
Ans. The bullying started at the age of four, when Katsuki's quirk developed. He began to look down on Izuku for confidence and constant reminder that he's the 'better' one, and that he doesn't need help from anyone, especially Izuku since he was quirkless.
2. Attack on titan fans = When was the first time Eren became a titan, and why did he?
Ans. The first time he became a titan was when he killed his dad; Grisha Yaeger, when he turned Eren into a titan. He was meant to put an end to all the titans, but instead later on in the future, he wanted to commit ominicide and end the future generations of Paradis and Marley. (still brought peace anyways :shrug:)
3. Death Note fans= How did Light find the Death note? Who was the first person he killed with it?
Ans. Light finds the death note in his school ground, when Ryuk (the previous owner of the death note) dropped it into the human world. From what I remember the first person he killed was a mass murderer...
answered three over eight questions.
~may it help~