I'd love you to really think about that. Demons have power over people, but Jesus has power over demons? So when possession happens, Jesus is just busy with other matters (or maybe relaxing in the heavenly sauna)? And what did the pigs do to deserve that fate, by the way?
There is really no point trying to explain anything to someone who has not desire to understand anyway.
There is a very good reason why God does not intervene to prevent anything bad from happening in this world. To do so would be to prop up the devil's side of the issue. At the end of the day, we are all going to be caught in the act of being ourselves...either upholding God sovereignty and his laws....or not.
The pigs were in all probability going to be killed and eaten anyway. According to God’s law swine were unclean animals, and not only were the Israelites forbidden to eat their flesh or to use them for sacrifices but even to touch the carcasses of swine made a Jew unclean. For them to raise swine was therefore a flagrant disregard of God’s law. Since they had no business raising swine even for commercial purposes they could well consider this destruction of their herd as a just rebuke.
Also, it might be reasonably argued that one man is worth more than a herd of swine, especially since swine were supposed to have no commercial value among the Jews. So no fault can be found with Jesus’ permitting the demons to enter into the swine.