‘Monster’ is a flexible term. It can of course be used literally, to mean some sort of hideous otherworldly creature – the kind which appears often in video games.
Every so often though, the term can be stretched out and draped over another human being, someone so decidedly evil that no other word will fit.
We see a lot of those types of monsters in games too – Bioshock’s Andrew Ryan is a great example; driven by greed and corruption, he built walls between himself and the people he saw as “parasites” profiting from the work of the elite.
Source: Eurogamer Eurogamer Q&A: Gaming's Greatest Monsters