寝返るねがえる
JLPT: +
to change sides, to double-cross, to betray

Kanji:
Godan verb with ru ending, Intransitive verb
Example Sentences

こいつ、さっきまで俺の相棒だったのに、もう寝返りやがった。
This rat, up until just now he was my partner and now he's gone and turned on me.