Who ever said anything about selfishness?
The worst trolls are the ones with ideology, who believe themselves to be working on behalf of a greater power. To take a relatively innocent example: Tesla Trolls will harass, dox, and even SWAT "Tesla Shorts", in defense of their beloved company.
So lets circle back again: Why are these Tesla trolls trolling the internet? Because they're fighting to support their company, and it seems to be an effective tactic for intimidating and silencing critics.
Same things happen with Apple Fanboys, XBox fanboys, etc. etc. Trolls "fight" on behalf of the powers they've given their allegiance to. And of course, Democrats and Republicans are big ones too... but also subgroups: Antifa, Black Lives Matter, Proud Boys, etc. etc. They all have trolls, they all will harass and demean you if you disagree with them online.
Any "serious" internet discussion will devolve into troll tactics these days. Why? Because its effective. Apple vs Epic Games is just the latest bit, and we're certainly going to see internet trolls start to weaponize forums to bring people to their side.