Augustine of Hippo

As a Christian judge, you must play the part of a loving father, you must show anger for wrongdoing but remember to make allowance for human weakness; do not indulge your inclination to seek vengeance for the vile acts of sinners, but direct your effort to the cure of the sinners' wounds.… There is also that well-known example of forbearance on the part of holy David, when his enemy was delivered into his hands and he spared him, an example which shines with greater luster from the fact that he had power to act otherwise. …

Letter 133 ↗

Ambrose of Milan

What a virtuous action that was, when David wished rather to spare the king his enemy, though he could have injured him! How useful, too, it was, for it helped him when he succeeded to the throne. For all learned to be faithful to their king and not to seize the kingdom but to fear and reverence him. Thus what is virtuous was preferred to what was useful, and then usefulness followed on what was virtuous.

On the Duties of the Clergy 3.9.60 ↗

Bede

And he said to his men, "May the Lord be merciful to me," etc. David is shown to have refrained from killing Saul, despite Saul's most hostile pursuit of him, for two main reasons: first, he remembered him as his lord and as having been anointed with royal chrism. Here, primarily, he venerates the kingdom and the anointing with due honor, which indeed he always knew symbolized the ever inviolable kingdom of the Lord and the spiritual anointing. …

Commentary on Samuel ↗