In my opinion, the key difference between cats like Dirtstripe and Ashfur is that the former was stopped from doing evil, while the latter kinda always changed his mind on what he was doing. (Do bear in mind that I'm not defending him, I'm just trying to make sense of something as inconsistence as StarClan).

Like, for example: Darkstripe tried to kills Sorrelkit with deathberries to cover up his meeting with Tigerstar, and he fully intended to do so; she survived only because other cats discovered it on time. Ashfur tried to kill Firestar with the foxtrap to make Squilf suffer and whatnot, but for whatever reason, he changed his mind inmediately when he saw him bleeding and went back to camp to get help. He had no motive to do this (It's not some part of a grand conspiracy with Hawkfrost because his role in PoT was decided much later, the Erins had told us), he just got cold feet, I suppose?

Many of Ashfur actions while alive are like that. He wanted to do bad stuff, but didn't, out of his own (admitedly very inconsistent) will. Darkstripe, on the other hand, was fully willing to do bad stuff and to see them through, he just sucked at being evil and other cats continuously stopped him.