Oh I know that, didn't stop D&D putting it on screen though.
'Wanting the audience to remember a character as they used to be' is awful reasoning. 'Remember when Mr White was just the friendly local chemistry teacher, wouldn't it have been lovely for the audience to be able to remember him that way?' Fiction is full of characters who become monsters, so what?
I don't see how it cheapens the red wedding either, it's not like she's getting her life back.
To address your first point concerning Catelyn, yes lots of fictional characters become monsters. However, this particular fictional character's entire storyline was about how she would do anything for her family. In the books, this includes nearly hanging Brienne (a show favorite). We're not meant to like Lady Stoneheart.
Walter White is different because he reaches a point in his life where he can't go forward. He has terminal cancer. He's beaten down. He feels like he has to make a change, and that change turns him into a horrible person.
Catelyn Stark is as far as we know a loyal and dutiful wife from a very strange family. She makes some boneheaded political maneuvers and gets her entire family either killed or displaced in one way or other, and she dies. So the next logical step is that...a character who briefly met her daughter brings her back with red magic so she can hang Freys and others people who are just walking along the Riverlands?
That's a huge stretch, and not the least bit satisfying for me. The only Frey I want to see dead is Walder Frey, and I don't think we'll get that scene.
I feel that the need for northern revenge is most of the reason they put Sansa back in Winterfell, but once they looked at the surrounding conflicts, they couldn't give Sansa a whole lot to do except cry and get rescued. Hopefully I'm wrong on this.
The red wedding is an event that changed not just the Starks, but the entire landscape of northern Westeros and the Riverlands. We are meant to feel that there is no retribution for what happened that night. There is nothing that can be done to exact some measure of peace. Arya even says that Joffrey's death made her feel nothing. So when one of the Starks sets foot in their home again, and it is really
their home, it will really mean something.