I'm going to try to give you some insight into the activity of most demons, and what they are actually out to try and do with most of us, etc, but that sometimes doesn't go exactly according to plan. Let's take the man with the legion, and why Jesus sent them (those demons inhabiting that man) into the pigs, and why the pigs immediately rushed over the precipice to die for a minute, and why it was actually an act of compassion upon those demons by Jesus? Jesus sought this man out, the man with the legion, and he didn't do this with hardly anyone else, and made quite the journey/trip out of his way to do it, etc. When a demon, or a group of demons, first makes a decision to target somebody, the first thing they do is get inside the person for what they think is going to be for them (at first) somebody that is going to provide them with a lot of very great fun, and much entertainment, at first anyway, etc, but, when the fun stops being fun or entertaining anymore, then they usually try to get that person to off themselves or die so they can then get released from that person, and go elsewhere to do it with or to somebody else again, etc. Most of the time they are successful with most people and it's not a problem for them, etc. But, every once in a while, they can get stuck there with somebody for a while and then it becomes a torment for them until that person dies, etc, and this is what had happened with the man with the legion, these demons desperately wanted a release from this man by that point, because they couldn't get him to kill himself, and couldn't kill him, etc. Well, Jesus seeks this man out, knowing full well what was going on with; both that man, and the demons in him by that point, etc. Well, when these demons saw Jesus coming they knew exactly who Jesus was, and that he had full authority to send them all into oblivion if he wanted to, etc, and this is why they entreated him, and practically begged Jesus to send them into the pigs instead, and is why the pigs immediately rushed over the precipice and died, so that these could finally be released and were free to go elsewhere now after that, etc, finallyget there long sought release from this man that they originally targeted (but greatly underestimated, etc). They knew Jesus wasn't going to kill the man, or let them kill the man (something they had been trying to do with the man, or get that man to do for a very, very long time now, but couldn't) so, anyway, that's why they practically begged Jesus to let them go into the pigs nearby, and Jesus allowed them to do that instead of "something else Jesus could have done to them", etc, and it was actually Jesus having compassion on both them (those demon) and the man that those demons were inhabiting, since it was the man that Jesus was after, etc. Anyway, this is the activity/purposes of most demons with most of us, but it can backfire for them if their attempts to make the person dead after it is no longer fun anymore doesn't succeed fully for them, because once they get in a person, they don't have the power to get released from that person until that person dies, etc, which doesn't always happen always, after which they are the ones that are in torment for a while until that person dies, or until someone like Jesus comes along, and they are able to get released another way, etc, which is what happened with this man with the legion.
God Bless.
