What many political scientists and historians say is that Nixon was focused on two things that ended up making him do stupid things to destroy himself. First, he was obsessed with getting even with his enemies. Second, he was very concerned about his place in history. So this mean that he would be very aggressive in attacking anyone and anything that he saw as a threat.
In the context of Watergate, this meant that he would aggressively attack those who were investigating. He saw them as enemies and believed that the investigation would tarnish his reputation. Ironically, this led him to really destroy his reputation by acting to try to cover up the scandal and to attack those who were investigating it.