Mortals sin meaning human beings. What about other animals who kill? Don't they sin too or is it just their nature?

Animals kill to protect or survive, they have no intent, just instinct. Humans have intent. So no, its not fair to label animals as sinners in my opinion.

In the 19th century Americans started moving west and three institutions went with them. There was vaudeville, traveling entertainment. There was lyceum, traveling education and culture. And there was the itinerant preacher, offering a new style of preaching called "hell fire and brim stone". It was very entertaining, only loosely based on scripture, and pastors didn't even try to compete. Instead they switched to preaching public morality and philosophy. Eventually an entire generation grew up not knowing the first thing about the religion they claimed to believe. That is why most Christian churches don't teach doctrines, and most members don't know what they are supposed to believe.

Sin, singular, is the absence of spirit. Adam had a spirit but it died and so he was unable to bequeath a spirit to his offspring. That is why everybody is born in sin, not because Mommy and Daddy did the big nasty. That is why a savior had to die in our place and make it possible to create a new spirit for us.

Sins, plural, are mistakes, such as throwing a dart and missing the bullseye. Christians are baptised into Christ's death, and dead bodies don't sin, so sins are not an issue for Christians. There is no more forgiveness. Either you believe in Jesus and are saved or you don't believe and are lost. 

Read a chapter of Proverbs every day. Proverbs has 31 chapters so you can keep your place by just looking at a calendar. There is no religion or nothing in Proverbs and you don't have to believe anything. Just read every day. When you are comfortable with that, then read the bible from Romans to 2 Thessalonians over and over until you start to remember what it says. That is the part that applies to Christians. You will notice that most of the religious crap you hear from random preachers isn't in there at all. It is made up.