As far as I can tell, this introduces a new singleton tag quake which has no other questions right now. It is my understanding that (regardless of whether or not the quake tag should be kept) such a question should remain tagged video-games, even if it has a specific tag.
A specific, discoverable tag like quake is a good thing. This is also consistent with our habit of tagging questions with the name of the work, or of the writer for written works.
(It used to be that tags with a single question were automatically deleted after a while, but this is no longer the case, so there is no reason to eschew good tags.)
Tagging with media is also useful, because many people are specifically readers, or movie aficionados, or gamers. In an ideal world, I would support tagging every question with the medium that applies to it. However, it is unrealistic to require a constraint like this on every question, and the tag engine doesn't support making such tags compulsory or even strongly recommended. Furthermore, there would be a very large, unwieldy number of questions tagged novel or movie. So I don't push for systematically using those tags. Instead, we mostly use them to distinguish questions about movies and written works in the same franchise.
The situation with video-games is different. It's a more specialized interest: there are gamers who are interested in the background stories of video games (which are often SF) but aren't frequent readers or moviegoers. video-games has 11 followers — not much, but not insignificant either. So I'm in favor of keeping video-games and continuing to use it.
Feel free to roll back bad retagging.
Past that, there are several issues here. If the story is both a movie and a video game (Resident Evil might be an example), I'd only use the video-game tag if you wanted answers specifically about it. But for something where the story is only a video game, I'd use this to fill out the tags if I hadn't reached the max, but if you're already at 5 relevant tags, you should potentially drop this if there is a more relevant tag. (And these rules, in my opinion, hold true for both movie and tv-series.)
Tags only exist so someone can search. If the editor is getting rid of tags without good reason, they're hurting that feature. Please add the tag back to the post.
I don't think there has to be a single rule that is obeyed for all media types. Questions about novels, tv shows and movies are common, so a compulsory novel or movie would, for most purposes, be annoying, adding unnecessary noise. Useful for taxonomy fans and data miners, but not for most users of the site. :)
Video game questions are rarer, and as Gilles said, possibly of interest to non regulars. However, this raises the questions - would other non-standard media types deserve their own tag? Should we radio for the War of the Worlds or the Hitchhikers Guide skits? Should we add a musical when some zombie musical hits the air?
I say no. I think video-games is, specifically, a medium that has a following that is distinct enough from "regular" SF and Fantasy fans, and so should remain tagged, even if we don't treat other media types the same way.