The murder of Tywin Lannister
Following the saga of misunderstood and bullied characters in ASOIAF, lord Tyrion Lannister is the one who really takes the blame for everything.
Historically speaking, in the middle ages, babies born with dwarfism were either given away or killed at birth. The very few exceptions were kept in high society and their families carried that as a spot of shame. Dwarfs often became circus attractions and were considered a burden to their families.
Under that same premise, Tyrion was created. Lord Tywin evidently didn't get rid of him as a baby because of his high status. As lady Joanna died while surrounded by a maester and likely several ladies, Tyrion was pronounced well and strong. With so many witnesses, the right opportunity to get rid of that child wouldn't come easily and so Tyrion lingered, but his lord father, who simply couldn't stand laughter and mockery, saw his son as a constant reminder of his beloved wife's death and of mockery. That being said, the opportunity to blame Tyrion for the boy Joffrey's death was much too good to be ignored. Once Tyrion leaves that cell, he goes confront his lord father who is in the loo and there, Tyrion shoots him.
Strangely enough, Lord Tywin's death isn't just a shot in the gut. He stinks up for days and the fact that he is caught in the privy and can't get up to confront Tyrion brings to light some evidence that, although Tyrion shot and killed his father, he might have saved lord Tywin from a far more humiliating fate.
In A Game of Thrones, Eddard XV, lord Varys tells lord Eddard Stark that "the Martells still brood on the murder of Princess Elia and her babes." and once the reader is introduced to Prince Oberyn, it becomes evident that they still do. Prince Oberyn is repeatedly described as sentimental, partially mad, a fierce lover and a fiercer fighter. He is also said to be extremely knowledgeable in poisons and the stories of his travels to study even in the Citadel are well known all over Westeros. His own brother describes him as "deadly, dangerous and unpredictable".
When Prince Oberyn arrives in King's Landing, he makes it plainly clear that he was seeking justice, and not only for the Mountain, but also for the man who "gave his orders". He didn't trouble himself into hiding his true intentions. He wanted blood and vengeance.
When Prince Oberyn talks to Tyrion about lord Tywin, he said: "Your father may not live forever." Something about the way he said it made the hairs on the back of Tyrion's neck bristle. (Tyrion IX, ASOS) We all know that George R.R. Martin has never been afraid to give his readers foreshadows of his intentions. That was one of them.
But even before Oberyn used a poisoned spear to fight the Mountain, he had a history with poisons. When he was 16 years-old, he was challenged into a duel for being caught in bed with Lord Yronwood's paramour. On that duel, each man took a cut. While Prince Oberyn's healed, lord Yronwood's festered and he died. That event granted him the nickname "Red Viper".
In the same book, Tyrion talks to Maester Pycelle and is taught about "widow's blood", which was a poison used to shut down a man's bladder and bowels until the man "drowns in his own poisons". GRRM always liked to introduce the reader to the actions that were in the horizon.
Surely, being in the privy isn't evidence that he was poisoned, but the detailed descriptions of his body's rot while in the Sept undoubtedly is. "The vapors rising from the corpse were making Pycelle's eyes water". "The King's Hand was rotting visibly. His face had taken on a greenish tinge, and his eyes were deeply sunken, two black pits. Fissures had opened in his cheeks, and a foul white fluid was seeping through the joints of his splendid gold-and-crimson armor to pool beneath his body." "Tommen turned his head and doubled over, retching." " The septons were the first to see, when they returned for their dawn devotions. They sang their songs and prayed their prayers and wrinkled up their noses, and one of the Most Devout grew so faint he had to be helped from the sept. Shortly after, a flock of novices came swinging censers, and the air grew so thick with incense that the bier seemed cloaked in smoke. All the rainbows vanished in that perfumed mist, yet the stench persisted, a sweet rotten smell that made Jaime want to gag."
The rot from lord Tywin has a strong association to the Lannisters corruption as well. Ser Jaime's hand was heavily associated with corruption. That corruption linked the hand that pushed Bran and that fiddled under lady Cersei's skirts. Once that hand was removed, Ser Jaime became a better character to the reader. In the case of Lord Tywin, he was rotten inside. His corruption caused him to be a terrible father to all his children, especially to Tyrion. That same corruption is represented through his lies, lies that they were wealthy, lies that they were strong.The Lannister pride affected by the smell of his corrupt, rotting body.
Another point to consider, is that Prince Doran in AFFC said: "I have worked at the downfall of Tywin Lannister since the day they told me of Elia and her children. It was my hope to strip him of all he held most dear before I killed him, but it would seem his dwarf son has robbed me of that pleasure." Not the pleasure of death, but the pleasure of a downfall.
Although the death of lord Tywin as a slow process doesn't seem like something Oberyn would do after he so clearly demonstrated he was after blood, the slow undermining of his power and authority while causing people to laugh at him does sound like the proper revenge. The issue remains, however, that what Tyrion did to his father, unknowingly, for sure, might have been the best thing to preserve some of his honour.