I got the same issue after upgrading from a macbook to an imac.
On the macbook SupCom2 worked flawlessly, but after migrating to the imac I got the same issue: Steam hanging while trying to launch SupCom2, and require a force quit to exit.
The same happening when trying to verify the integrity of the game cache or trying to delete the local files.
Also reinstalling Steam and downloading again SupCom 2 didn't work for me (as well as the usual mac os x troubleshooting: permission repair, cache clearing etc.)
Drop a ticket to Steam support hoping to get a reply from them.