sql server - Unable to delete the database user as the user is already logged in currently. error 15434 -

i have created user sagar in sql server 2012 database , mapped xyz database. after time deleted xyz database , don't need existing user sagar also. when try delete user sql srever management studio. getting following exception

could not drop login 'sagar' user logged in. error:15434

user can deleted after killing session identifying session_id of user.

select session_id sys.dm_exec_sessions login_name = 'sagar'  kill 51  --51 session_id here, may different id 

now can delete login executing below query (or) using sql server management studio options.

drop login 'sagar'