So I absolutely love the Mantraps. They’re one of the most viscerally terrifying Doctor Who monsters I’ve seen in a while, especially combined with the way they’re shot - semi-perceived through a bubble, or dead on coming out of the elevator. That slow zoom-out shot with Lindy standing helpless as four of them surround her gave me the shivers. Everything about them is primed to make you feel disgust and fear.
And that neatly ties into the answer to the question of “Why didn’t the Dots just kill them normally?” and the answer is simple - the Dots aren’t killing because of some sort of population control, or malfunction. The Dots are killing because they viscerally hate the Finetimers. This is pretty close to I Have No Mouth And I Must Scream, and AM did some frankly horrifying things to those humans because of its hatred.
So why kill them outright and fast, when instead, knowing everything that you know about the Finetimers, you have the ability to genetically engineer (or potentially just attract) creatures that would not only totally horrify and disgust the Finetimers, it would kill them in a really visceral (and let’s not forget, LONG, it looks like those slugs take an awfully long time to eat you and it feels like they’re conscious for pretty much all of it) and humiliating way. The Mantraps are silent, they’re methodical (because they’re working to The Dot’s plans) and you don’t know they’re there until it’s too late. Combine that with the frankly Geiger-esque vaginal horror that they embody (which ties nicely to the Mummy who turns up - who knows, is this Mummy all their Mummys? Are the Dots returning them to the parents who sent them away in a weird sick metaphorical manner?) and you have a recipe for a horrifying, humiliating, undignified death, which in The Dot (and maybe the viewer’s) opinion, they deserve.
But it serves a double purpose, because we don’t know the full extent of the Finetimer’s racism, we don’t know how truly broken their world is, until the end. So we sit and we empathize with them because yea, it would be disgusting and horrifying to die to a Mantrap. It works to make us sympathize, and it puts us in The Doctor’s shoes by the end of it. If you take a humanist approach to life, as The Doctor does (and in my better moments, I hope I do) no matter what someone is, or what they do or believe, nobody deserves to get killed by a Mantrap. We all do have intrinsic value, and this episode does SO much work to show us that The Doctor, above everything, is loyal to the value of preserving and saving life, wherever he can. And it isn’t enough to save these people. It’s powerful, and the slugs are a part of that. If it was just the Dots BOOM HEADSHOTing people, it just wouldn’t land for me.
