Tinker, Tailor, Soldier, Spy

Although Jim Prideaux thought Bill Haydon was his closest friend, Smiley's exposure of Haydon as Karla's mole showed Prideaux that Haydon had betrayed him to the Russians and thereby almost got him killed. Because of all Prideaux had gone through because of Haydon's betrayal, Prideaux exacted revenge by assassinating Haydon with a sniper rifle.

Haydon was an all round bad guy, to say the least. In addition to his betrayal of Prideaux, Control, and the Circus generally, Haydon also seduced Smiley's wife, Ann, in order to better keep tabs on Smiley. Haydon was a treacherous fellow, indeed.

Just before the assasination there was another party-scene where both Mark/Prideaux and Haydon/Firth shared a long look between them. I thought that look was more like a look between lovers rather than friends but I could be off here
What I base that on is the return of Mark as was earlier discussed in the movie as a favour to Haydon. There was no real reason for the russians to return Mark and I wonder if Mark would have kept his mouth shut unless they were something more than "just" friends. Since Mark must have known who the leak was long before the end since he only told Haydon where he was going in the beginning. A friend who betrays you is no longer a friend but someone you love....then it start to get complicated.
The assisination itself could be for two reasons, that he wants to get even for the betrayal or out of mercy.
Based on my interpretation above I rather think he did it out of mercy, to avoid the questioning, as a member of the circus he probably knew what the questioning would entail.
Haydon seduced Smiley's wife because Karla thought that Smiley was the one they had to look out for, if they could disrupt his watchfulness by making trouble in his private life. They would potentially move the focus for Smiley to just see a hated rival rather than a potential spy.
As far as I can remember, Haydon even said that himself in a round about way.

We agree that Haydon targeted Smiley's wife because of the detrimental effect it might have on the brilliant Smiley's ability to identify Haydon as Karla's mole. I also agree that Haydon was bothered enough by his betrayal of Prideaux to use his influence with Karla to get Prideaux repatriated.

I confess that I have never inferred from Le Carre's novel or either film the existence of a sexual component in Haydon and Prideaux's relationship. In fact Haydon has always struck me as ragingly heterosexual. Maybe I'm just naive but I didn't see any such indication.

Prideaux had been so badly injured, both physically and psychologically, during his captivity he could no longer function in the field and had been reduced to being a teacher and coach at a second rate prep school. I thought that was plenty of motivation for Haydon to do the right thing (for a change) and convince Karla to return Prideaux.

Le Carre makes clear that Bill Haydon had a reputation for being as attracted to men, and vice versa, as he was to women. Le Carre also hints that Jim Prideaux and Haydon may have been lovers as well as friends. Interestingly, the novel implies but does not actually say that it was Prideaux who killed Haydon at Sarratt but does not specify whether death was caused by a broken neck, as was depicted in the 1979 miniseries, or by gunshot, as depicted in the 2011 film.
In both the miniseries and the film, the extent of Prideaux's post repatriation disability is underplayed. In the book, though, Prideaux has a frozen right shoulder and his wounds are still discharging. In short Le Carre describes a far more profoundly disabled man than either film shows.