That is why I try my best to read the entire Bible and instead of reading into it what I want to say, I use exegesis and read it for what it really does say. It does not make me popular in some Christian circles though because the Bible is really clear on many things some of which is not politically correct nowadays.
I'm fairly sure you don't follow everything the bible tells you to do either, otherwise you'd be posting this from the inside of a jail cell. But, luckily for you, preachers and religious scholars have reinterpreted many parts of the bible to conform more closely to the culture of the times
I am assuming you are referring to things in the Old Testament such as polygamy, animal sacrifice, war and such? Just because the Bible give a historical account of what people did does not condone what they did.
I read the words of Jesus and try to follow them. I can understand God's relationship with man based on the stories in the Old Testament as well. But I won't pick and choose the parts I don't like. In understanding other cultures I can get a glimpse into why things were allowed (polygamy because so many men were killed in war at that time). Jesus was our ultimate sacrifice for our sins so the animal sacrifices are not necessary anymore thankfully!
And what about the flood with Moses to take one arbitrary example? Is that not mass murder and genocide? Who forgives God his sins?
The flood was with Noah. Is it wrong for God to judge us when we disobey? How bad was the world that he decided to have a world-wide flood to destroy it? Perhaps we do not have any idea how far humanity had degenerated to at that time. I do know it grieved Him to flood the world though but I imagine a cruel and evil world where there was human sacrificing, raping of women, brutal murders and so on. The groups of people that were "judged" or God allowed to be killed by war or by His hand were non-repentant and often involved in very evil practices such as child/human sacrifices in the Old Testament.
As for you question "Who forgives God his sins?" God has no sin. We can question His decisions but we are not omniscient and we do not know all things like He does. The one thing I do know is that I do not want to get on His "bad side" though! What shocks me is that people have no regard for Him but the Bible predicts this too. It says that the the last days "will be just like the time of Noah—everyone carrying on as usual, having a good time right up to the day Noah boarded the ship. They suspected nothing until the flood hit and swept everything away." Who fears and respects God anymore? God is loving and kind and wants all of us to repent but He is also not to be trifled with and if we do not repent He will judge us.