I am pretty sure that many of us come across of situations when a killed session by 'alter system kill session' command did put the session in 'KILLED' status and never released the session for a long time on the database. It could be due to the fact that the session would be rolling back the ongoing transaction.
Whenever we are in such situation, we generally try to find out the OS pid (on UNIX OS) associated with the killed session (which is a bit difficult task, as the killed session paddr in v$session changes while the addr corresponding value in v$process does not), and kill the associated OS process with 'kill -9' command on the OS level.
I have found the IMMEDIATE option with the 'alter system kill session' is more useful as it writes the following information in the alert.log file after killing the session and also try to finish the things at the earliest possible to close the session from the database:
Wed Feb 10 11:02:39 2010 Immediate Kill Session#: 515, Serial#: 36366 Immediate Kill Session: sess: c0000001be20d9f0 OS pid: 14686
As you see, it writes the time stamp when the session was killed, and also gives the associated OS pid of the killed session in the alert.log. As per Oracle documentation, 'Specify IMMEDIATE to instruct Oracle Database to roll back ongoing transactions, release all session locks, recover the entire session state, and return control to you immediately.'
alter system kill session 'sid,serial#' IMMEDIATE;