Whatever redeeming qualities they had seemed to have disappeared after Chrysalis snapped everyone out of being reformed. All three villains, even Cozy Glow, have rejected friendship and at this point none of them can be reasoned with anymore.
Or did you forget what Fluttershy said in “Daring Doubt?”
Fluttershy was seen cheering after the villains were turned to stone. It’s clear she doesn’t give a shit about them. What she meant to say was “everycreature deserves kindness… except Chrysalis and the others”.
If any one of the villains, but only one of them, went back to Twilight and wanted redemption, would anyone believe them? If Cozy Glow was given the chance she’d pretend to be reformed just to create another evil plan. Chrysalis on the other hand wouldn’t want anything to do with friendship and wouldn’t even pretend.
Tirek is probably the most redeemable unreformed villain. He’s the only villain to have had standards, questioning Cozy Glow’s plan of draining all of Equestria’s magic (although this is likely due to his traumatic experience with Twilight and her friends). He also seems to be capable of honoring a deal, such as when he gave Chrysalis back her magic even though he did not want to. Tirek also seems to care about his family, particularly his grandmother. Cozy Glow doesn’t care about anyone but herself (and possibly Tirek) and Chrysalis doesn’t give a shit about her former subjects anymore.
