John Calvin Commentary Psalms 109:4

"For my love they are my adversaries: But I [give myself unto] prayer." — Psalms 109:4 (ASV)

On account of my love they have been opposed to me. The Psalmist had already solemnly declared that his adversaries, unprovoked by any injury he inflicted on them and without any just cause, became, through mere diabolical rage, his most implacable foes. Here he confirms the truth of that declaration by saying that he had been their friend. For there is far more merit in showing kindness to an enemy than simply abstaining from doing what is evil.

And from this we may perceive that the influence of Satan must be terribly powerful when he takes the hearts of men captive at his will. For nothing can be more unnatural than to hate and cruelly persecute those who love us. To love he also adds deeds of kindness, meaning that it was his aim to secure their goodwill by outward acts of beneficence.