As an HR, among your main roles is to ensure that all grievances are resolved to avoid legal issues. But what can drive you to file a complaint legally?

Every firm has a policy of conduct and every employee should abide by it. Any unprofessional behavior they may cause harm to the firm or its employees should be handled legally.

Dishonesty of Employees and unethic thing which can't be ignored

as per company policy

dishonesty of employees and been accused of stealing 

When the employee is unable to solve the problem

when the employee is at a position wherein no dicussion or compromise can work out. 

When then the company cannot handle the issue or nothing can be down no matter what in the deem legal counsel

When the issue at hand is beyond the control of the firm's legal counsel and parameters; or when the employee is at a position wherein no dicussion or compromise can work out

its all about the countries constitution,rules and privacy.