Majin Buu killed Babidi because Goku had given him the idea (in their fight) that he didn't have to follow Babidi's orders.
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Goku killed buu (buu in his final form as "kid buu") with the spirit bomb
Because his evil side was awakened thanks to Babidi, who released Majin Buu.
Majin Buu and Super Buu (Super Buu Gotenks Absorbed and Super Buu Gohan Asorbed)
Majin Buu
All forms of Buu can talk, expect possibly Kid Buu. Majin and Fat Buu have relatively low intellects which affect their speaking; Kid Buu was not shown to talk, but did make noises such as grunts.