After being fired, he was at his bitterest and even deleted all his colleagues' contacts.

bitterest 是形容词 bitter 的最高级形式。bitter 本意是"苦的"，引申为"痛苦的、愤恨的"。 bitterest 表示"最愤恨的、最痛苦的"，用来形容极端负面情绪的状态。