She had done well.
Her Master was a legitimate magus.
He was in his thirties, had a medium build, and had few noteworthy characteristics.
He had no will to fight, but dreamed of victory. He was a man that only waited for the other Masters to kill each other.

The man had not trusted Caster.
He treated Caster coldly as she was a superior magus, and he insulted her for being inferior to other Servants.

She gave up on him after a few days.
She acted as an obedient Servant and filled the man's conceit.
In short, she made him use his Command Spells for meaningless things.
She told him he did not need the Command Spells.
She made him believe that she would be faithful to her Master even without the Command Spell.

It was his fault for believing in her.
The Master used his third Command Spell for a meaningless thing, and was killed by Caster at that moment.
It was easy.
She did not like the fact that the contract with him still existed, so she killed him using the Rule Breaker.