When one, in the past, wrote a nasty letter, people would describe this as a "pen dipped in vitriol," vitriol being sulfuric acid, an immensely caustic compound that worked well in ink, but burned the hell out of anyone who touched it directly. The chemical burning property led people to connect it directly to the idea of vehemous anger. Older inks amounted to essentially carbon dissolved in water, and lacked the permanence of vitriolic inks. These older inks were the baseline comparison, since they were less offensive to the touch and quicker to fade away.
Vitriolic ink is now very much a thing of the past, with more effective mixes of dyes and pigments providing the same permanent coloration without the risk of chemical burns, and for significantly less money. That and most text these days is no longer on paper, but in electronic signals, like emails, blogs, and webpages.
sulfuric acid, however, has not gone away, and aside from being the primary acid in batteries, a rust remover, an effective pH lowering agent for fertilizers, a laboratory acid for dissolving compounds, part of pharmaceuticals, and it's used with iron to produce ferrous sulfate, a preservation agent in foods. Especially processed foods. In fact, engineers claim that a nation's sulfuric acid production is a good baseline to describe how industrialized it is.
A common stereotype of internet addicts is that they sit around all day eating processed foods, namely "Cheetos," a puffed corn snack with a powdered cheese coating, and, you guessed it, iron sulfate for preservation. Such people are also said to amuse themselves by producing angry messages on the internet about everything they dislike. Their pen may have been replaced by a cursor, but the vitriolic dipping remains.
So going by these assumptions, they've become a vitriol-processing device. Chemical vitriol goes in, information vitriol comes out. I found that hilariously ironic.