I could answer this a few different ways, but I think I will give the answer I like best, which paraphrases my late mother (God bless her soul). She was both loving and wise.
"Everyone has darkness and light in them. The darkness is tempting because it tricks you and makes itself seem more attractive than the light. If you feed the darkness, darkness grows. If you feed the light, light grows. If you feed enough of the light, it will snuff out the darkness."
This in essence is the struggle of human kind. It is in our nature to sin, to want to do bad things, some more than others. We have the choice to follow those natural desires, or to fight them. Those who do bad choose to give in to temptation instead of fighting their desires. Doing the right thing all the time is the much harder path to travel, in life.