Why did Cleopatra poison her husband?

It is generally assumed that Cleopatra poisoned her second brother/husband. Her reasons were completely self-serving. With her brother out of the way, and no other brother alive to be a co-ruler with her, (Egyptian law said that a queen had to have a co-ruler)

she was free to name her own co-ruler and she did. It was her infant son Ptolemy XV or Caesarion.

This way Cleo had all the authority and power for herself as the baby would not care one way or another about politics. Also, by having her brother dead, she also freed herself from the possibility of him ousting her from the throne as her other brother did a couple of years before.