Imagine having an alcoholic father who abuses your mother, and has absolutely no interest in you, and leaves bastard children every where he goes, proof of his infidelity, like stains.
Imagine wanting to learn sword-fighting, and they hand you over to a man whose face was badly burnt by his own brother when they were kids, a man whose self-professed only love is killing things.
Couple this with the fact that you are in fact a product of incest, the incest of twins no less, and their parents were cousins too. Even biologically, you do not have a good start in life.
You of course, do not know. You think that the obese, womanising alcoholic is your father, and not the golden knight.
But imagine everyone whispering all the time, wondering about your mother, asking are you an abomination, do you need to be killed.
But you dismiss this rumours as nothing more than treason, not truth. The king is your father. You are a Baratheon.
Now take all of that, and imagine people telling you all your life that you are a Prince, that one day you will be King, that you will rule the kingdom, and imagine how difficult it would be to stay level-headed, not to get arrogant with encouragement like that.
Now let's take a look at what you've done so far.
You hire a man to kill a crippled boy. That is inexcusable, yet you were only trying to please your father, who clearly stated that the boy would be better off dead.
Later, you lie, and the consequence is that a dire-wolf ends up dead.
You lied, because you were beaten by a girl, and that hurts, that rankles with you. You lie, because no one has ever bothered to instil a sense of right or wrong in you. You are the Prince. You must always be right.
Other character's lies started wars and killed men, yet yours results in the death of one dog, and you did not even ask for the animal to die, that was your mother's work.
You systemically abuse your bride-to-be.
Well, you physically abuse her- slaps and such, though you get your kingsguard to do the worst of it - and if the finest knights in the kingdom don't say a word against it, then how wrong can it be?- and you're just following the example of how your father treated your mother.
You tell her that you will rape her, yet you never follow through, you never even kiss her.
Already, that makes you better than your father. You are still wrong, inexcusably wrong, but in context, you are not as awful as they make you out to be.
Time passes. You are now the King. The man below you has admitted he's a traitor, and he has dared call you the product of incest. He even conspired to get you off the throne.
Your mother says that you should show mercy, but you cannot be weak now. This is your first act as king and besides, the man deserves death.
The same man swung the sword himself for lesser charges than this. He had killed men for abandoning their post in fear for their lives. He has done much worse. The outcome should not be unexpected. You call for his death.
After that, generally, you are horrible and cruel, selfish and vain.
This does not make you the worst villain in this world. There are others who do far worse than this. Don't forget, human sacrifice, child rape, torture and cannibalism are just some of the few things that other characters in Westeros have done.
You call for your father's bastards to be killed.
This is an order to kill children, infants, and again, it can not be justified but can be explained at least.
These children are the stains of your father's infidelity, and in these times where your own parentage is called into question, your own right to the throne, it is annoying that these bastards look more like your father than you ever did. This is partly why you kill them. You also did it for your mother, who must hate them.
After that, you do not do much else. Certainly, the abuse carries on, a slap here and there, the odd blow if she is especially insolent, and of course you are an awful King, one of the worst that ever was- yet not as bad as your father's predecessor, who burned people alive at court. Never as bad as him.
You are vicious, disrespectful and cowardly. These are all traits that had been nurtured in you since you were small, and if you had a better upbringing, would have been lessened if not eradicated.
Perhaps the worst sin that you commit is the murder of a prostitute.
Your grandfather despises whores, and your uncle knows that, and yet he arranges for you to have one. You only see the murder as a message to your uncle. The lives of others have become meaningless to you now. Thousands are murdered in the war in your name, and other men go out and murder and pillage and rape, and you killing a grand total of one does not seem too serious to you.
That is it. That is the list of your sins. Murder, lying and abuse.
You are villainous, irredeemably so, but then again, you were 12 when all of this started. You are perhaps 13 now.
No child has ever shown great understanding of consequences, and you are no different, just on a larger scale. No child has ever been handed the power you have either and expected to cope well. The throne turned good grown men like your father into bitter things, it ate them away, so what did they expect would happen to you, the arrogant selfish little child?
In a world of slavery, war, rape, cannibalism and torture, how can anyone say you are the worst? You are just a golden child who lost his way.