Waterbenders become stronger when fueled by emotion. They draw power from their emotions, and so by being angry, they are capable of unleashing more powerful attacks, but a possible result would be that they lose control over their bending.
This is shown many times throughout the series. When Katara was a novice bender, her temper would often unleash her waterbending prowess, but she had no control as to what her powers affected. She destroyed icebergs completely, among other things. Later on, her skill and control improved, and she was able to fuel anger while still being in control of her bending. She showed this when she sent a wave of water toward Jet, as well as when she effortlessly bloodbended a Southern Raider captain.