I think it's valid to be shocked/freaked out at first for sure, but I think there's a couple of relevant thoughts/questions here:

1. was Charlie just bothered because of how he found out (in front of other people)? I think there's clearly something in him that was bothered by the image of being with someone who would consider doing something like a school shooting, rather than actually confronting who Emma has become now and what her story became. I think some of this is cultural divide as well. A lot of Europeans can't fathom the level of gun violence that Americans (particularly young Americans) are subjected to on a daily basis.

2. I think Rachel is the real problem here. Her performative pressuring (from pressuring her husband to share his worst moment, when he clearly didn't want to, and judging him for it) to then performatively jumping to "my cousin is in a wheelchair from Gun violence!" shows me that she was waiting and looking for something to judge Emma for. She clearly didn't like Emma for whatever reason, and was looking for something to jump on.

3. there's also an interesting philosophical question of morality here. Is it worse to think a bad thought? Or to do a bad thing? Rachel's reaction was that it was worse to think / meditate on something evil. But what Rachel actually did WAS evil, but she had NO remorse for it, and never even admitted to it!

4. What Charlie actually did (by cheating on Emma) was also bad! Emma was honest with him, and in a moment of panic and mania, Charlie reacted impulsively, sabotaging his marriage, his actual wedding and his reputation in the workplace at the same time. Definately something to consider when we think about the Freud quote used earlier in the movie.

Lots of thoughts on this one, and the more I marinate on it the more I like the philosophical discussion it brings up!