I’ve read through a good chunk of this thread and I think most people are correct in that you should initially be facing this head-on, particularly with the problem dude in person in front of the group. As a secondary action, I’d also recommend tightening your availability for full group activities afterward if it doesn’t resolve itself. What does that look like? Maybe one or two gaming sessions with the full group anymore, but the couple of chill dudes in the group can be separately invited to play something with you, maybe a new, different game. If the problem dude tries to put you on blast for doing this in some way, you have the ammo of “I’ve tried to give you a chance to be less toxic. You’ve refused. I’m trying to maintain friendships with the people who are considerate about my feelings. I’m still making an effort with you and you keep showing me that you’re a bad friend towards me. How much of your bullshit do you think is a reasonable amount for me to have to put up with before I’m justified in walking away?”
I’m sure I don’t need to tell you this, but if you’ve seen him talking about the others behind their backs, and you’re not online with them as often as the others, then he’s almost certainly talking about you behind your back more than he does with everybody else. And any decent person in the group is aware of this and will respect you for speaking up. Just don’t do it yourself in your subgroup with the chill guys. If they bring it up, just say something like “guys, let’s not worry about them, let’s just focus on having a good time together right now.”