Why do some people go through life getting away with wrong doings. they always have an excuse for their bad behavior and its always believed?

To start at the end, it's not always believed. You don't believe them and I don't believe them, and I know a whole lot of others who don't believe them.

Now the why; most people who behave this way don't know how not to. Like congenital liars, something early in their life put them on this path and they wouldn't know how to do otherwise. They will connive and lie even when there is no reason for it and they can gain nothing by it. They can really only get by with it for a limited time for a limited group of people, because people do catch on. Once they are on this path, they never trust that other people are not doing the same.

We've all had to be associated with this type and there's nothing you can do to change them. You can only minimize your contact with them; don't trust them even when the circumstances or they seem to be different this time, they're not; and never become emotionally invested in them or their ideas or activities. I try to be polite and respectful whether they've earned it or not. Like getting the best behavior from children, you ignore as much of the bad stuff as possible and notice the occasions when they do something good.