What the heck does the company (Polymarket) have to do with any of this?

There's nothing wrong with a prediction market. There are a lot of reasons why they're helpful in judging the probability of future events more accurately than any single analyst, and are therefore a net benefit to society, no different from newspapers and stock markets.

These are just individuals making criminal threats. They're the bad guys here. They don't work for Polymarket or anything. Your comment is like saying you can't imagine how anyone could go to work at the Olympics after Nancy Kerrigan got struck by Tonya Harding's ex-husband. Something criminal happened, but the Olypmics are not the one responsible.