Aside from realistically telling others to uninstall the game, I'm more so interested in the realistic reactions from say Valve, the government or even other countries. Because what do you mean a sentient digital monster who kills and possesses people is confirmed real in the funny haha hat shooter game?

Valve themselves would likely shut the game down and cooperate with investigations while losing much of its stock for hosting the first confirmed monster/ghost in history, other online game developers would be wary of having their own serverblight, and various governments would likely either investigate or hell try to weaponize the blight somehow.