Answer:
A cruel prince may be better for his subjects in the long run
Step-by-step explanation:
The idea is that a cruel prince will have a feared ruling, so crime will be lower. If the Prince is not like that, or if the Prince is too merciful, people will not fear the Prince, therefore they might act outside of the law more often. Also, a cruel prince will more likely be seen as merciful if mercy is given from time to time in a cruel setting than if the Prince is always merciful but crimes keep happening.