root of evil; source of trouble; cause of misfortune
Refers to the underlying cause or origin of a problem, often with a nuance of something that will lead to future trouble if not addressed. Common in formal or written contexts.
That policy ended up leaving the seeds of future trouble.
To eliminate the root of the problem, fundamental reform is necessary.
That decision became the root cause of the later conflict.
An easy compromise can also invite a new root of trouble.
火種 is a more colloquial metaphor for a potential trigger of trouble, while 禍根 is a formal term for a deep-seated cause of future misfortune.
Compound of 禍 (calamity, misfortune) and 根 (root). The combination directly yields 'root of misfortune'.