The angrier you get the harder you fight. Players with the Badass talent get a bonus to combat skills when they have bad morale (which can range from "bad mood" to "miserable and depressed").
As a prerequisite, the PC has to have the melancholy trait.
Effect: According to ghchars.pp, this appears to be a +1 bonus to each combat skill per 20 points of negative morale, and a -2 penalty at 20+ positive morale.