The toxicity of the (overall) internet is imo down to how everything now have to be so politicised, both from the left and the right.
Go back to late 90s and early 00s internet and I don't remember that level of toxicity. Sure there were still internet trolls on forum and in chatrooms but it wasn't politicised. Trolls were there just to annoy other internet users and admins/mods.
The problem started in so-called Web 2.0 with the rise of Twitter and similar, making the internet accessible to the lowest denominator. It got even worse from the early 2010s onwards when Twitter could be used on a phone (as an app), so any time, anywhere.
It's not just the internet either. It's in real life and on the TV too. Everything is, just, politics. It encroaches into Hollywood, pop videos, the workplace, university, parties, traditions e.g. Christmas, dating and so on. In the late 90s, when I was still an adult but a young that is, I remember people simply got on better.