Good and evil are relative concepts. The idea of an absolute good or absolute evil... is illogical.  Any absolute form of good or evil would be such only from the limited range of perception and comprehension of the human mind. On top of that, such understanding and perception would be bound by the natural laws and order of our world.

So, what IS "evil"? It would be VERY simple to label anything that differs from us as being evil. Or anything that opposes our desires or intents or existence. But that is an evil as understood by a being of limited perception and limited scale of time. The Abyssal plane is older than our religions and our sciences. And yet we, in our tiny minded ways, think to label it as evil. Laughable.

Now, I'm not saying the Abyssal elements are not anti-thetical to our existence. Nor am I saying that there is no harm that occurs when contact is made or leaks occur.  What I am saying is that it is entirely possible that the damage done is completely seperate from actual intent.

For example, our magi and scientists attempt to send someone to the Abyssal plane to retrieve some sample... be it in a container or in them. It is a known _fact_ that humans cannot survive for long within the Abyssal realm without serious protective gear. There is no "air" to breath. The laws of physics there are completely different. The reality as perceived by two explorers into the space become completely random and chaotic. Put simply, humans who travel to the Abyssal realm are assaulted by the very nature of that space. Through no actual ill intention.

To top it off, while we only see the damage done to our explorers, have we ever stopped to consider the damage done to the Abyssal plane? What vast swaths of land have we accidentally raized because of our lack of understanding? What if our physical forms causes massive and far travelling waves. Our Order could very well be like a killing field for all Abyssal life forms that come into contact. WE could be the violent and destructive interlopers.

So if we look at it from that point of view, an Abyssal being that enters our world... is assaulted by forms of energy, concepts, structure, and pressures that is not native to its space and could very well be destructive and existence threatening to them. Perhaps this is why "demons" of old are portrayed in such a damaged and decayed or corrupted state. Because that is how WE SEE THEM. Their energy and form as perceived by the material plane is that of beings set afair... made of lava... screaming and thrashing... essentially suffering.

This leads to the next point: possession. Any port in a storm, right? If you cannot exist in this space, then survive within someone who is of this space. So Abyssal beings do the only thing they can... they dematerialize and they establish psychic links with humans who are open to such things.

But their presence within the human mind is a poison. They cannot survive here even if they hid within the body of a human native. The very existence and definition of the space rejects what they are. Not as something evil..  but as something too different to co-exist in this space.

So, just as I do not believe all humans to be evil, I do not believe all Abyssals are evil. Their actions must be viewed within the context of what they are dealing with. And if given the chance to exist within our realm without being under constant assault, would these beings be able to communicate with us? Understand us? What could we learn from such opportunities?

So why do possessed people do evil things a good deal of the time? Suffering.

The Abyssal being is suffering. Even within the mind of the host, they are being bombarded with physical consistency and screaming. Entry within a human body and mind does not grant it immunity, it only brings down the intensity. To top it off, the Abyssal realm is assumed to be a place of chaos. And that those from that world would want to take over this one.  What if Order itself was blashemy? What if not explorers, but their equivelant of priests or magi are the ones coming through? They would be arriving in a world locked in apparent death. They could very well be trying to bring things back to life.

This is, of course, all speculation on my part. But I choose to not assign the label of Evil to the Abyssal realm just because circumstances dictate they are.