Which attacker category might have the objective of retaliation against an employer?

The answer is: Insider

In this context, Insider refers to attackers that are part of the organization's members.

They can be the people who felt that they've been treated unfairly by the employers and initiate an attack to the employers out of revenge. In general, insiders tend to find it easier to find sensitive information from the company and tend to do more damage compared to external attackers.