I actually enjoyed online interactions with random people back when I was still playing online games in 00s. Still remember the dude who totally whooped my and my friend's arse in Dawn of War just to let us live, build back up a bit and lead a war on two fronts against his Necron hordes (we almost got him, but ... Then he got the revive thing for his Necron Lord before it was nerfed. Damn.)
'Spose it might be just me getting old, but cool interactions like that felt increasingly sparse when I was trying to play MP with randos, all the way to me ... Well, not doing that anymore.
Oh yeah, there are tonnes of good memories even playing with random people. Just that story makes me crack up when I remember it just for how outlandish it was.
But for toxic behaviour, I think it's SSDY, well, at least in MP FPS games. From Quake 2 with people claiming everyone else were hacking homosexuals with prostitute mothers, to CS where I got to hear that (along with nice porn sprays mind you), to DoD with everyone thinking them playing "Wollt ihr den totalen Krieg?" over voice chat was witty and so on.