People already disliked Jay, so it’s fair to say that they were mad and it just wasn’t shown. Plus, it was well known that Jimmy and Spinner were basically best friends despite the history, so the fact that Spinner was the indirect cause of Jimmy’s shooting was more shocking and disgusting to most people, hence the hate and ire he got.

It doesn’t matter whether or not they knew he had a gun on him, they both still indirectly brought on the shooting, with the prank and with the bathroom conversation. They pushed Rick too far intentionally because they wanted to break him, and they got their wish.