Yes, by the strict Jain standard, it is a sin/karma-producing thought.

But the degree matters:

• Thinking about self-harm creates bad karma through the intention.
• If the thought has already stopped and you are not accepting it now, then it is less severe than actually doing it.
• Doing the act is far more गंभीर than merely the thought.

So the strict answer is: yes, it is sinful in Jain ethics, but less intense if it remained only a thought and you rejected it.

In Jain Dharma, the right response is pratikraman, repentance, and guarding the mind.