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The command issued does not actually kill the session. It instead requests the session (likely via the SGA) to terminate - i.e. kill itself/commit suicide.
If the session is stuck waiting on a synchronous kernel call for example, it cannot respond to the kill request. In some cases, this means that as the session is unable to continue (as it is waiting on a synchronous call that will never return), it cannot be successfully instructed to terminate.
In which case the typical action is to instruct the kernel to physically remove the (stuck) executable process instead.
You can try disconnecting the session. It is available from oracle 9I onwards.
ALTER SYSTEM DISCONNECT SESSION ('SID','SERIAL#') IMMEDIATE;
ALTER SYSTEM DISCONNECT SESSION ('SID','SERIAL#') POST_TRANSACTION;
Hope this will help. Please refer to below link for further information on this.
Thanks and regards,