They wouldn't have to show it in gory detail like they did with Talisa, I actually would have preferred the slicing of Cat's throat off screen. But to me her death just didn't have the impact it had in the book. Cat is a fighter, she wouldn't just stand there and wait for someone to kill her if she hadn't lost it. The stuff I described showed that she'd gone mad. And the shot of her standing there waiting for someone to hop in from offscreen, kill her and hop back offscreen looked kind of goofy IMO.
What bothers me too about Talisa is that they even brought her. As careful as Cat is before the wedding, making sure they don't do anything to insult Frey, she wouldn't have allowed her to be there. The showrunners like portraying the Starks as even dumber than they are.
Just like with Sansa, we get it, she's a naive moron, but on the show she says dumb shit like asking if her family could come to her wedding, she ist not that stupid.
Edit: Another thing about Cat: why didn't she go straight for Walder? All she needed to do was crawl to him under the table, she was right there, 2 feet away from him, instead she kills his wife. Yeah, they love showing how dumb the Starks are