It's hard to give up bad deeds because of the conditioning of the mind. Bad habits condition the mind in a way that they become hard to give up. On the contrary, if you keep to the five precepts for a long time with care, it will be hard to break them because of the conditioning of the mind.
The closer you get to Nibbana, the harder it becomes to do bad deeds. In other words, the more you purify the mind, harder it becomes to do evil.