He treated his subordinate cruelly.
I can't put up with his cruel treatment any longer.
そんな不当な仕打ちを受ける理由はない。
There is no reason to receive such unjust treatment.
彼女は会社で不当な仕打ちを受けた。
She was subjected to unjust treatment at her company.
He endured that cruel treatment for many years.
I can't believe he'd do something so cruel to a friend.
待遇 describes formal treatment or reception (e.g. at work) that can be good or bad, whereas 仕打ち is personal and always negative.
Compound of 仕 (し, continuative form of する 'to do') and 打ち (うち, continuative form of 打つ 'to strike'), literally 'doing and striking'. The negative nuance likely developed from the idea of striking or harsh actions.