In Jain Dharma, the thought of killing someone is a very faulty and harmful thought. It is a form of himsa in the mind.

In Jain ethics, violence is not only in action, but also in speech and thought. So if a person deliberately entertains the intention to harm or kill, that creates pāp-bhāva and bondage of karma.

However, Jainism also understands that:

• a sudden bad thought that comes and goes is not the same as
• deliberately holding, enjoying, or planning that thought.

So:

• A passing bad thought = faulty, but one should reject it immediately.
• A wished, nourished, or planned thought of killing = highly sinful in Jain view.

The right response in Jain Dharma is to:

• become aware of the thought,
• stop it at once,
• do pratikraman and repentance,
• cultivate maitri, karuna, and ahimsa.

So yes, such a thought is definitely considered faulty and spiritually dangerous in Jainism, especially if it is held intentionally.