Leave is a privilege and not an entitlement is a dogma on which the leave policy formulated in many companies. Therefore, if your leave was approved only for 6 days instead of 12 that you had applied for, then there must be some reason to it. We members of this forum are outsiders and do now know the inner dynamics of your company.
As a compromise, you were ready to forego your salary for the extra leave and wanted to apply for Leave Without Pay (LWP), however, that was also turned down. This again was the discretion of the approving authority.
Just because you have sufficient balance of leave does not it becomes ground for the approval. Various factors like work culture of the company, whether you have any standby manpower, volume of workload in your department, your relations with the superiors, how many days in advance you had applied for leave, whether company follows any leave plan etc play important role in taking decision.
Not approving leave cannot be called as vendetta. This is the most unsuitable word that you have used to describe the situation. You may check the dictionary for the correct usage of the words before you write your posts in the public forum.
General comments: - Employee had applied for leave. His leave was approved partly. When he overstays, HR quickly sent letter to the employee. He had clearly specified the reasons for the leave and was in constant communication with the approving authority. In that case, what was the need to show impetuousness and mar the pleasure of his brother's marriage? The show cause notice for unauthorized absence could have been given on resumption of duties by the absented employee. It is this tendency to display misplaced promptitude makes HR untrustworthy and causes mental estrangement not just with HR but with the company itself!