"Who have been the most toxic people in your life, and why?"
This is just an example off-hand. A person who was a friend of mine for approximately a decade. He was part of an inner-part of a circle of about 6-7 close friends. We all started to drift apart around when I was 22, but he remained close to me.
"In what ways were/are they toxic?"
He was as a manipulative back-stabber. At first I perceived his behavior as personal confidence about things that were bothering him about our other friends. That slowly grew into an awareness that he would attempt to play friends against each other to be the most important; to raise himself up in the eyes of others.
I once shared an apartment with him and another friend. He would complain about us to one-another, and violate personal space behind the backs of my friend and I. His pretext would be trumped-up arguments about behaviors and rumors to do so. I was unfortunately a willing participant in this, because the other roommate was slightly unstable, and displayed outlandish and obnoxious behaviors. I had not known the person well for too long to have accurate background context on what that person was doing.
He was seeing a girl but refused commitment, despite conveying every sign it was inevitable. They were tacitly partners. He would lead her out of our apartment in a charming manner. Then he would shut the door, and proceed to complain about how annoying she had been. He would rotate through women while seeing her, and compartmentalize them all from one-another.
One time, he had the main girl he was seeing and a side-girl come to the same party. I guess he was curious how skilled he was at deception. I presume he wondered if he could have them both in the same place without either knowing his relations with them. He did manage to pull off the act. Following that, there were numerous instances where he would try seducing the girlfriends of his friends.
"Were they sociopaths?"
I imagine so.
"Something else?"
A bit of a coward when it came to confrontation.
"How long did it take you to acknowledge their toxicity and make the decision to either remove them from your life or create a firm boundary?"
It was difficult for me to reconcile a somewhat self-conscious person I had known in my early teens with the blossoming of their Machiavellian desire to be the center of social circles. After I split from my apartment with him, I frequently spent time with him. We would go out and party, or just spend time at his place with his roommates, and do whatever entertained us.
It was when he began talking behind the backs of both of his roommates that I started putting everything together. He would talk behind his one roommate's back to turn him against the other they lived with. Everything became so obvious at once. I was bothered that it took me so long to piece it together. But then I seen the full picture—he talked behind everyone's back. So, I had a couple conversations with some members within the new circle of friends we had at university, and it all confirmed what I thought. It should have been apparent, but I was fooled by the appearance of confidentiality and years of camaraderie.
I stopped speaking to him any longer, and I warned my friends about how he was. Suddenly everyone started seeing things from a new perspective, sharing anecdotes of their own, etc. It must have gotten back around to him that I was doing this. I ran into him by coincidence twice in public, and he avoided me (pretended like he did not see me, started walking another direction). He's dead now, so I don't think about him much anymore.