So what would have happened would alexander have ececuted him or spared his life
Alexander pressures Darius to name him as heir and abdicate. And then he'll probably be killed shortly after.
I think Darius, if captured, will stay alive at least until Alexander and Stateira's wedding (and that of Haephastion and Drypetys)This most likely. Or maybe he holds him as a hostage for his entire life.