As there is no point in asking “why” (because no action is involved and asking such changes nothing), I will not ask why people are sinful and evil. Rather, I understand that people are already sinful and it has become part of our nature due to our ability to recognize our “self” entity. We chose our self over God, and this ultimately led to the most extreme sin-our turning away from our Creator.
Satan has been there the whole time, having turned away from God before Humanity even came onto the scene. He continuously influences us to constantly turn away from God in a repeated fashion so that we may never come into union with God. Satan does this so as to assume authority over God and to take what it Gods by conquest using humanity as his warship.
The degree to which evil has spread into the world now is beyond me. The best representation of it is to say that even virtues that we claim to be “good” can be twisted for evil purposes. The pleasure of sex is twisted into a constant and insatiable lustful craving of the body; a human being can make oneself feel bad simply so that others will take pity on him and then make it happy; and worse yet, a father abuses his family by threatening to remove his support from them if they do not do his bidding.
Speaking from personal experience, evil occurs when your family keeps secrets from each other because of God-knows-what… This is something I will never understand.
So then, how does one do good in a world that scarcely believes in God? One sure-fire way to fail is to use verbal reasoning. The only thing that will convince people is acts of faith and compassion to (quite literally) bring the will of the Father to Earth. When someone asks you why you do the things you do, then you can explain to them in full the influence God has in your life. Ultimately, it comes down to living by example for others. Humans, in our current state, cannot understand God because of our limitations in our consciousness. As such, those who believe must live lives that demonstrate God’s goodness, forgiveness, love, mercy, and discipline as best we can. Telling someone to read a book may lead to inspiration, but your actions as a man of Christ speak volumes that would take longer to read than the time one has on this Earth.