How many Kingsguard betrayed their king?

Question

I know of Jaime, Barristan and 2 turncloaks, are there any more?

Answer 1

Explicit betrayers

• Ser Gyles Greycloak is considered a turncloak for reasons yet unknown.
• Sers Olyver Bracken and Raymund Mallery abandoned King Maegor and joined Prince Jaehaerys when he rose in rebellion. Sent to the Night's Watch for breaking their oaths by the new King Jaehaerys. Broke their vows to the NW as well and rose up in mutiny.
• Sers Erryk Cargyll, Steffon Darklyn, Lorent Marbrand betrayed their King (if you consider Aegon II the rightful King) and defected to his sister Rhaenyra Targaryen. All three died in their Queen's defense.
• Sers Criston Cole, Arryk Cargyll, Rickard Thorne, Willis Fell betrayed their Queen (If you consider Rhaenyra the rightful Queen) and defected to her brother Aegon II. All except Ser Willis Fell died in defense of their King and his children. Willis Fell survived and served as Lord Commander for Rhaenyra's son Aegon III's seven.
• Ser Mervyn Flowers is widely considered to have had a hand in murder of Queen Jaehaera Targaryen, Aegon III's first wife and conspired against the King and royal family presumably on behest of his brother Lord Unwin Peake. He also murdered his Lord Commander when he came to arrest him on King Aegon III's orders.
• One unnamed Kingsguard of King Daeron I who betrayed his King and dishonoured his cloak by surrendering when King Daeron was treacherously attacked and murdered by the Dornishmen at a parley. Three of his sworn brothers including Ser Olyvar Oakheart died defending the King in contrast, while Prince Aemon fought on till he was taken captive.
• Ser Barristan Selmy betrayed the rightful heir of his King by taking pardon from Robert Baratheon and serving in his Kingsguard rather than staying true to his King's heir Viserys III.
• Ser Jaime Lannister, betrayed his King Aerys II and murdered him. Also took pardon from Robert Baratheon and served in his Kingsguard rather than staying true to his King's heir Viserys III.
• Ser Borros Blount betrayed his King Joffrey Baratheon by surrendering and handing over Prince Tommen Baratheon into the hands of Tyrion Lannister's men.
• Sandor Clegane betrayed his King Joffrey Baratheon by deserting his post during Battle of Blackwater Bay.
• Ser Arys Oakheart broke his vows of chastity with Princess Arianne Martell and betrayed his King Tommen Baratheon by handing over Princess Myrcella into the hands of the Dornish conspirators (A conspiracy he had joined himself) who plotted to pit Myrcella against Tommen.

Sort of Betrayers

• Ser Orivel the Openhanded is considered to have been a coward for reasons unknown. If he showed cowardice at a moment when the King needed absolute loyalty and courage, that can be considered a betrayal. After all, courage and loyalty of a Kingsguard are supposed to be ironclad.
• Sers Jon Tollett and Symond Crane were given a choice to take the black or the axe for their perceived oathbreaking by failing to defend King Maegor to death (Though there was no proof, there were rumours that it had been the Kingsguards who had killed the late King Maegor to protect the realm from his cruelty). They chose the Black. Their sworn brother Ser Harrold Langward was accused of the same charges but he chose to demand a trial by combat instead and was slain as a result.
• Ser Lucamore Strong broke his vows of chastity by secretly taking three wives and siring a multitude of children. Since the vows of chastity exist in order that the KG must not have any divided loyalties between their families and their King, you can say he betrayed the King and was rightfully punished (Gelded and sent to the Wall).
• Ser Gyles Belgrave was accused of either directly or indirectly poisoning his King Aegon II. He denied poisoning the King but admitted that since he had failed in his duty to protect the King, he should be sentenced for an oathbreaker. Lord Hand, Cregan Stark agreed and executed him himself.
• Lord Commander Ser Marston Waters did kind of betray his King by executing a coup although circumstances are unclear. On pretext of the rising influence of Prince Viserys' Lyseni in-laws with help of the hand Thaddeus Rowan, the Lord Commander of the KG took control of Aegon's Hill in Aegon III's name (The King was a minor) and moved to arrest Aegon's sister-in-law Larra Rogare and her family. The King and his brother refused to give Lady Larra up and closed the gates of Maegor's Holdfast. Though Marston did lay a secret siege to Maegor's Holdfast while his King and the royal family suffered inside without food or water for 18 days, he refused to order his men to attack the King he had sworn to protect, he instead chose to beg the King to open the doors and to reason with him. And when Prince Viserys unraveled the wider conspiracy, King Aegon ordered Marston to prove his professed loyalty by arresting the real culprits. True to his oath, he obeyed the orders to arrest the men who had apparently fooled him. During one such arrest, he was murdered by his own sworn brother Ser Mervyn Flowers. To this day, it is debated whether Marston was in on the conspiracy and turned on his fellow conspirators once he saw the plan failing or was the simple Knight used by politicians (Former hand Lord Unwin Peake was chief suspect) for their own ends.
• Prince Aemon Targaryen, the Dragonknight may have betrayed his brother and King Aegon IV if the rumours regarding him having an affair with the Queen Naerys and giving the King a cuckold's horns were true.
• Ser Terrence Toyne betrayed King Aegon IV by breaking his vows of chastity with King's own mistress and paid for it with his life.
• Ser Osmund Kettleblack betrayed his King Tommen Baratheon by breaking his vows of chastity with Queen Cersei Lannister. Ser Kevan Lannister was contemplating sending him to the wall if he pleaded guilty, alternatively he'd have to face Ser Robert Strong in a trial by combat which was just another name for a death sentence.