I think I know what you mean by sensing an oppressive presence. Whether imagined or real, I have always had such a sense when in some places (in certain sections of New Orleans and San Fransisco, for example).
As to the demonic nature of so-called aliens, many people who claim to have been abducted speak of this.
Whitley Strieber described it well in "Communion" (he has since recanted).
Then there is the Trickster quality of the so-called aliens. Some people have said that nothing about them is as it seems.
I think it may be overly simplistic to assume the Trickster is a demonic quality exclusively. Jung's description of the Trickster archetype occasionally puts him on the "good" side of the ledger.
That said, the trickster "aliens" seem to lie an awful lot. Because of this, it is probably smart to be wary.
When I was talking about demons lying, I was focused like a laser, on one issue, and one issue only. That is if you ask them if Jesus has come in the flesh? I am not sure, but this may be the one area that they can not be deceiving. I just don’t know.
As far as fallen angels go, Satan obviously had to lie to the other angels, about them becoming like the most high. The angels had never heard a lie before. It was new to them. They had a choice to make. I figure God must have communicated to Lucifer, that he was was close to crossing the Rubicon, from which there would be no return. Obviously, they didn’t heed the warning. After the fall, did some of the fallen angels wish they could return to the good graces of God? I don’t know. Maybe yes, but it was too late. They were irrevocably committed forever, to follow the path they had chosen.
Now, you may ask, could one of the righteous angels, suddenly decide one day, that he has served God long enough, and now wants to serve Satan? Theoretically, I think it’s possible, but I think it is 1,000% improbable. They have seen the power of God, up close and personal.
Evidently, the fallen angels are pretty unnerved, by the thought of hell fire 🔥 but they should have thought about that before they fell. Did God tell them about the lake of fire? 🔥 Who knows? They haven’t been there yet, so they don’t know how bad it will be. But I think it is infinitely worse than anything they could imagine in a million eons. The fallen angels in Tartarus, know exactly how bad it is. When the creed says Jesus descended into Hell, I think it was to Tartarus. He didn’t preach a second chance. He announced to them, that now, their doom was assured. There would never be another chance for them. I suspect the condemnation of Jesus is still reverberating through Tartarus, as they remember the glory they shared with God, and threw it all away. Truly a mystery.