I want to preface this by saying that this is not a defense of Shawn. He was a terrible partner and a terrible person. Behaving so badly on a reality show that your real life employer fires you should make you look in the mirror. But his faults have been talked about ad nauseum.
I feel like Shawn's terribleness is letting people gloss over how horrible Nikki was too. She was argumentative, nasty, and hardly ever took accountability for her faults. From the get go, she seemed to have no issue embarrassing him publicly. I feel like Shawn's humiliation at the alter is undersold. Imagine standing in front of all those people and having your soon-to-be wife verbally overlook you like that. Everybody there would have known why. I don't remember her ever apologizing for that because I'm sure he felt like shit when it happened. She can claim that it was because of the way he was dressed but I guarantee that if 6'3 attractive guy was standing at that altar in the same fit, there would have been no confusion.
She also came into the first group dinner with completely legitimate grievances but it seems like she wanted to express them in a way that would humiliate Shawn as much as possible. Her opening statement sounded like something out of Real Housewives. Shawn played that situation well by letting her talk without interrupting because I think the nastiness of her words really hit and superseded what she was saying. That's why the group didn't back her up and was quick to come to Shawn's defense when she mentioned height. The height comment wasn't even bad but at that point they'd been listening to this woman shit on this guy uninterrupted for God knows how long. Similarly the Decision Day speech was planned for maximum humiliation and when Shawn dished it back to her she was caught off guard and they immediately started chirping back and forth.
She can't take out what she dishes. The "Seattle 10, NY 7" is another perfect example. When Shawn explained the context of her grabbing his belly and saying "Look at you", it made a lot more sense why he'd snipe back at her. What really got me is the fact that Nikki didn't even remember doing that. She claimed during that reunion that the comment isn't in her character but no one watching the show would be surprised if she said that. Like you can casually shit on someone's appearance and completely flush it from your mind without a second thought but want to start a riot when someone says something about you.
Nikki was mean and shallow and if Shawn was a nicer person, we'd be talking about her in the same breath as Courteney. If anything she's lucky Shawn's such a POS because it's letting her fly under the radar.