cut (someone's) throat
To bring about or be the cause of someone's ruin, downfall, or failure. That discount supermarket is cutting our throats, there's no way we can compete with their prices! Jim set out to cut his brother's throat after finding out that he'd been swindled.
cut one's (own) throat
Fig. [for someone] to bring about one's (own) failure. If I were to run for office, I'd just be cutting my throat. Judges who take bribes are cutting their own throats.