I'm pretty sure they were deliberately engineered for maximum irony and agony. As mentioned and demonstrated, the Dots were perfectly capable of killing their users themselves. I think they deliberately built creatures that were so slow and cumbersome that anyone who wasn't constantly stuck in their own bubble could have easily outrun them, and whose ingestion process was intentionally drawn-out and painful.
Being so horrible that your own technology hates you is pretty much dooming yourself.
Yeah, but nothing was shown or said that indicated the AI had the ability to bioengineer giant slugs that would eat people in the order the AI wanted them to. Or how it became self-aware to begin with.
It's a question of means, not motive. Pretty much every other Doctor Who threat says where the non-cosmic villains got their stuff from or at the very least explained enough to let viewers guess. There's nothing else in the society that has anything to do with bioengineering so the slugs are unexplainable.
And there being actual intelligence behind the city's downfall takes away from the actual satire about social media. It'd be like having a message about looking both ways before crossing the street when the threat is from an AI hijacking cars to run people over.
And based on how their society functioned, it was implied hat the system controlling the Dots did everything. Engineering an organism might have been as easy as reprogramming a food production facility.
Then the show should have added a scene that shows the slugs being created.
Or in one of the chat scenes have a character mention the time they had the AI create a whole elephant that they had it kill and cook for them, thus showing their callousness (creating a living thing just to kill it despite being able to make artificial meat) and shows the AI has the capabilities to create large living organisms.
Or have one of the characters say they had the AI create a pet elephant for them that they have the AI take care of and have elephant noises from off-screen. Heck, since it's off-screen they could say they had the AI bioengineer them a unique pet, just have something to show that the AI actually had the capabilities to make slug monsters instead of just having The Doctor guess that the AI made them.
Every time the Daleks clone a new army (Power of the Daleks, Revelation of the Daleks, Remembrance of the Daleks, The Parting of the Ways, Revolution of the Daleks, and I'm certain numerous others) it's shown how they recreated themselves instead of just acting like they can generate new Daleks out of nowhere, I don't see why it couldn't be done here.
In both Space Babies and this episode an AI creates a monster. Except only in Space Babies were we told/shown that the AI had that capability.