As others said, the fact is that scumbags exist everywhere, and Brazil is certainly not exempted from this. Another fact is that Brazil is obscenely huge. The country is almost the size of Europe, and the number of online Brazilians nowadays is probably larger than some European countries' entire population. So, given that the ratio of scumbags is probably and roughly the same for every country in the world, the chance of anyone running into a scumbag Brazilian online is naturally larger than average, which tricks our biased brains into believing that "whoa, Brazil must be filled with scumbags".

And of course, the notoriously pervasive "stray dog syndrome", a sentiment that every Brazilian thing is inferior than its imported counterpart (including manufactured goods, culture and whatnot), and which is hammered into the unconscious mind of 11 out of 10 Brazilians from a very young age (myself included), only helps strength this confirmation bias.