Two-Face isn't consistently evil; Every time he thinks of a crime, he tosses his two-sided coin. It was only when the coin had a scratched side that Two-Face went ahead and committed the crime without questioning the outcome of the toss. This compulsion is a compromise between Dent's evil, two-faced personality, and his former, law-abiding self.
Over the years he has been shown to rely on the coin to varying degrees. Sometimes to decide whether or not to commit a crime, sometimes to decide whether or not to do something good, like save someone. He will even do whatever it takes to do good deeds if the coin demands it.
Since Two-Face has ugly scars on the left side of his face, he plays with it with garments that are styled and materialized differently on one side. Two-Face brings his obsession with the number two to his weapon choices, two .22 automatics, which he wields with extreme skill and intelligence. His psychotic obsession with duality and his crime plan centered around the number two often lead to his downfall.
In the questionable "Arkham Asylum: A Serious House on Serious Earth" one-shot, it was revealed that the doctors at Arkham had removed him from the coin and placed him on a die and then on a deck of cards. Not only does he have good and evil in rolling the dice, but he also has six different options and 78 in the tarot. Two-Face is totally dependent and can't even decide to go to the bathroom until he sorts the cards.
In the end, however, he got his coin back and also featured a rare instance where he had a different fate than the coin predicted: he freed Batman even though the coin landed the wrong way. During the countdown, until he is deported to "Hell's Planet" along with the other villains in Salvation Run, Pied Piper, and the Trickster offer to break him out of Belle Reve after defeating his escorting guards.