The fit for JC is better in San Antonio, the Hawks give up less, and salary fillers (Young, Bates-Diop) bring a solid mix of scoring and defense!!!
Also, Murray is a great defender himself with the offensive game Ben Simmons doesn’t possess. His skill set would balance out the things, like defense, that Trae Young lacks.
I’m also guessing San Antonio does this is they really want to commit to a rebuild, as Murray is making the team to good to tank (currently around the play-in).