--preparation
declare @users table (Userkey int, Username varchar(10), Manager varchar(10))
insert @users
          select 1, 'ABC', 'DEF'
union all select 2, 'XYZ', 'ABC'
union all select 3, 'PQR', 'ABC'

declare @accounts table (Userkey int, accounts varchar(10))
insert @accounts select 3, 'Test1'

--query
select * from (
    select a.*,
        case when b.Userkey is null then 0 else 1 end as HasAccount,
        case when c.Manager is null then 0 else 1 end as IsManager
    from @users a
        left outer join @accounts b on a.Userkey = b.Userkey
        left outer join (select distinct Manager from @users) c on a.Username = c.Manager) a
where HasAccount = 0

--to get requirement 2, use add ' and IsManager = 0'