Do not swear!
33 “Again, you have heard that it was said to the people long ago, ‘Do not break your oath, but fulfill to the Lord the vows you have made.’ 34 But I tell you, do not swear an oath at all: either by heaven, for it is God’s throne; 35 or by the earth, for it is his footstool; or by Jerusalem, for it is the city of the Great King. 36 And do not swear by your head, for you cannot make even one hair white or black. 37 All you need to say is simply ‘Yes’ or ‘No’; anything beyond this comes from the evil one.[a]
Eye for Eye
38 “You have heard that it was said, ‘Eye for eye, and tooth for tooth.’[b] 39 But I tell you, do not resist an evil person. If anyone slaps you on the right cheek, turn to them the other cheek also. 40 And if anyone wants to sue you and take your shirt, hand over your coat as well. 41 If anyone forces you to go one mile, go with them two miles.
What to do when we swear out of negligence
Saint Basil the Great, Epistles, Epistle 199, XXIX, in Church Fathers and Writers (2010), vol. 3, p. 301
The governors who swear to harm their servants deserve to receive a remedy ready to heal a disease which must be treated with the greatest care. Their treatment is of two kinds: first to teach them not to swear anymore and secondly not to keep their bad decisions. That is why if somebody had the initial intention to harm someone else, this one should prove his regret for the thoughtlessness with which he took that vow and he should not use piety as a pretext for following that vow. Herod did not have any benefit because he followed his vow, because for not rejecting it he made himself a murderer of the Prophet. The vow was forbidden once and for all and the caused disaster must be condemned even more. For this reason who made a vow should repent so that he may not try to make an even greater damage.
Search its absurdity and you shall see: if someone took a vow to take out the eyes of his brother, would it be good if he did something like this? Or if someone took a vow to kill?
Or if someone took a vow to disobey any of the commandments?
I have sworn and determined not to sin, but to keep the judgments of thy righteousness. (Matthew 14, 7; Psalms 118, 106).
As we have the duty to enforce the commandment by steady decisions, in the same way we should destroy and uproot by all means the sin.
Saint John Casian, Spiritual talks, 2nd part, The 2nd talk with father Joseph, Chapter XXV, 5-6, in Church Fathers and Writers (1990), vol. 57, p. 610
„(…) The pious men, the Angels and even the Almighty God often changed their decisions […] We consider that it should not be imitated that easiness of a hasty oath, made in the state of an emotional disorder.