Is it considered sinful and forbidden for a group of employees to leave their current company to establish a new company of their own in the same field, knowing that their departure will negatively affect the current company, and that they do not harbor ill will but rather seek their own benefit?

The employee is a private contractor, and the lease (employment contract) is a binding agreement that cannot be terminated except with the consent of both parties. Therefore, one must adhere to working with the company until the contract expires. After that, it is permissible to leave the job and start one's own business, and there is no harm in that, even if the company is affected.