In order to be "good" we must be making a choice - a choice based upon logical, higher-order thought as opposed to instinctive reaction/habit/lower level heuristic thought. Most of the time, we do not make choices - we act from habit, we react in accordance to the rules of lower-level thought. It is often difficult to make a choice.
People respond to incentives. Actions which are a response to physical incentives cannot themselves be considered good. The incentive for a good action must come from elsewhere. It is therefore not possible for us to force people to be good directly - but it is possible for us to use force to compel the lower order mind into creating the conditions in which thought and goodness are possible/easier.
For while responding to physical incentives is not good, it does not follow that we should constantly fight our lower level mind. It is not necessary for us all to be ascetics. Further, without certain social and physical conditions, thought itself is impossible.
So violence can make us good in the sense that; our actions are not totally (or even largely) based on rationality, that the conditions surrounding us affect our ability to be rational, that the conditions surrounding us are largely determined by other people and that unthinking people can be affected by both positive and negative (violent) incentives.