This was honestly mercy and the kindest option
Idt people realize but bullets hurt like HELL. They burn like crazy when hitting you. Light got shot like 5 times. He couldn't even stand anymore by the time Ryuk wrote his name down.
Regardless of whether Ryuk intended it as a mercy kill or not, this was honestly the nicest thing that could've happened to Light at this point. Either he slowly and painfully bleeds to death, gets executed by the Task Force or spends life in jail. Ryuk doing this let him die with SOME measure of acceptance to his fate.