Towards the end of his letter to the Galatians, Paul says: whenever we have the opportunity, let us work for the good of all, and especially for those of the family of faith (Gal 6.10). There is much wisdom in these words. I want to explain a little bit of what I understand from them.
I recently attended a wedding around which there was some considerable controversy. The parents of the groom did not approve of the whole arrangement, and so they were not present. During the portion of the ceremony at which the groom and bride were asked whether they take each other in marriage, etc., the groom was particularly emotional and was crying. I imagine it was at least in part because his own parents were not there.
The parents, I am sure, are convinced that they had ample reason to object to the wedding. As I said, it is a wedding around which there was some considerable controversy. But they did not act wisely in boycotting his wedding, if I may so express myself. In some short amount of time, whether a few months or years or whenever, they will get over their objections and protests, and they will want to restore the relationship between parents and child. One obstacle this, one which will remain written in the immutability of the past, is the fact that they were not present for the wedding of their own son. This is something that they will not be able to erase or undo, something they will regret on their death beds. They would have done better, seeing that the wedding was going to happen anyway, to set their objections aside and be there for their son to help him in whatever way possible.
This is at least part of the reason why we must be ready and disposed to do good to everybody, as Paul says. It is not worth living with regrets over words uttered, actions committed, etc., out of ill will. Whether the other person was a total stranger, or else a family member or even a fellow Christian, your malevolent act will eventually become an object of regret -- at least if you are full of the Spirit's fruit: love, kindness, gentleness (5.22-3). This will be an obstacle to your prayers. Evagrius the Solitary said, Whatever you do to avenge yourself against a brother who has done you a wrong will prove a stumbling-block to you during prayer. These words are certainly true in my own experience.
I regret the words spoken against friends in the heat of the moment. I regret the coldness and anger towards my brother in times of conflict or estrangement. There is no changing the fact that these things happened. What can be changed, however, is my disposition to do them again in different contexts in the future. Paul calls me to renounce all ill will, every act of angry disposition, and to seek only the good of the persons around me. Naturally this requires an amount of control over myself that I cannot naturally possess, and in the end what is required is the fellowship with the Holy Trinity which exists in love. But in fellowship with the Three, I can be transformed into their loving image.
At the end of the day the Bible teaches us that God is love (1 John 4.8) and that whoever claims fellowship with this God must likewise love. God did us good and only ever does good to anyone; this is clear if he is willing even to die for sinners while they are still enemies (Rom 5.8). Nothing else matters except faith working through love (Gal 5.6).