Side by side with the more official legal processes, the right of men to avenge their kinsfolk's deaths seems to have been more or less recognized and approved, at least by custom. Thus, Dame Anglides can give her son Alisander his father's bloody doublet and charge him as a duty to avenge the murder on his uncle Mark, La Cote Male Taile can appear at Arthur's court wearing his father's bloody coat and proclaiming his intention to wear it until his father is avenged, and Alisander's son Bellangere can at last avenge the deaths of both Alisander and Boudwin, all without a word of either legal or moral censure - indeed, with more than a hint of praise.
The important requirement seems to have been that such killings for revenge be done in the manner of fair and honorable combat; a revenge killing done from ambush, or perpetrated by a number of men acting against one, were spoken of with outrage and contempt. It must also have helped if the person slain out of revenge was generally disliked; Mark's eventual death at Bellangere's hands seems to merit praise, while Pellinore's death at the hands of Lot's sons merits censure, even though both are merely mentioned, not described, in Malory's text. Mark was recognized scoundrel, Pellinore was famed as a noble knight.
Mark was also killed in revenge for treasons that would be considered murders today as well as in Arthur's time, Pellinore in revenge for a killing done in plain battle. This suggests that personal revenge may have been an accepted way of getting at your enemy when legal means failed or were unavailable for any reason.
Probably the most notable blood feuds were the one between the sons of Lot and the family of Pellinore, which started early in Arthur's reign when Pellinore killed Lot in battle and which eventually resulted in the deaths of Pellinore, Margawse, Lamorak, possibly Dornar (Drian), and Patrise [who died of poison which a kinsman of Lamorak's meant for Gawaine]; and the one between Gawaine and Lancelot, which rose in the very twilight of Arthur's reign, when Lancelot killed Gareth and Gaheris by accident in rescuing Guenevere from the stake. It may have been such fueds as this that gave rise to the old saw,
[T]here is hard battle thereas kin and friends do battle either against another, there may be no mercy but mortal war,
even though Lancelot cites the proverb, not in reference to blood feuds, but in reference to the wounds his kinsmen have inflicted on him in the Winchester tournament, through failing to recognize him as Lancelot.
We can understand murder and violent death giving rise to such vendettas; it is more surprising to find that mere jealousy almost started at least one such bloodletting.
Sir Tristram achieved many great battles, wherethrough all the noise fell to Sir Tristram, and it ceased of Sir Launcelot; and therefore Sir Launcelot's brethren and his kinsmen would have slain Sir Tristram because of his fame. But when Sir Launcelot wist how his kinsmen were set, he said to them openly: Wit you well, that an the envy of you all be so hardy to wait upon my lord Sir Tristram, with any hurt, shame, or villainy, as I am true knight I shall slay the best of you with mine own hands. Alas, fie for shame, should ye for his noble deeds await upon him to slay him.
Considering the magnitude of the personalities involved, we breathe a sigh of relief that Lancelot nipped this affair in the bud.
As kinship and friendship was at the heart of these feuds, so, conversely, the parties involved might be protected from further reprisal by other kinships and friendships. Thus the position of Gawaine and his brothers as favorite nephews of Arthur enabled them for years to carry on their feud with Pellinore's family with no more than moral interference from other noble knights who disapproved of the way they were handling it. For instance, Tristram once met Agravain and Gaheris, told them,
[Y]e be called the greatest destroyers and murderers of good knights that be now in this realm," but refrained from chastizing them bodily for the death of Sir Lamorak: "Well ... for King Arthur's sake I shall let you pass as at this time.