“Is it always safe to kill sessions which are inactive and few days old , still is present in v$session .. sometimes session is running but shown as inactive ..”
It depends. Are the old sessions causing any problem?
If they are ad-hoc users or client-server application sessions: Slayer Mode [Active]
In case they can be part of a session pool from a “modern” (21th century) application, it can be somehow important. If the last call et is really high, maybe it would not cause bad effects to kill. In case of need, the app will rebuild the pool. But be more careful in this case and perhaps it would be better to talk with the app/middleware specialist before taking any aggressive action.