This question contains spoilers…
(view spoiler)[they were capsules in mara's body for inhabiting her abilities but jamie and stella managed to get it out of her body. what if the capsules stay inside her, is she going to get worse? (hide spoiler)]
If they had stayed inside of her, yes. They were implanted by Kells to make her 'safer'--to stop her from being able to exercise her ability, and that _did_ happen. But because her ability was a natural part of her, the attempt to "fix" what was never broken in the first place made her sicker, physically and mentally. The way I thought about it was like what often happens when you declaw a cat--because they can't use their first line of defense, they learn to use their teeth instead. Which is what Mara did when she began killing people with her hands.