Why did octavian have caesarion killed

He followed the advice of Arius Didymus who told him that "Too many Caesars is not good". (no joke I swear he actually said that) Hope this helps ;)

Well to answer that you would have to picture yourself as Octavian, octavian was the heir to Julius Caesar even though he was adopted, on the other hand Caesarian was believed to be the son of cleopatra and Julius Caesar, so he threatened Octavian' throne because Octavian was just caesar's adopted son while Caesarian was said to be his real son, so Octavian had him killed because he threatened his throne.