How can this person who so annoys or offends me be God's messenger? Is God so unkind as to send that sort across my path? Insofar as his treatment of me requires more kindness than I can find in my own heart, demands love of a quality I do not possess, asks of me patience which only the Spirit of God can produce in me, he is God's messenger. God sends him in order that he may send me running to God for help.
The Psalms are full of cries to God about enemies--but it was the enemies that drove the psalmist (for example, in Psalm 64) to cry. If he had had no enemies, he would have had no need of a Protector. God will go to any lengths to bring us to Himself.
"Think of him who submitted to such opposition from sinners: that will help you not to lose heart and grow faint....You have not yet resisted to the point of shedding your blood....The Lord disciplines those whom he loves" (Heb 12:3,4,6 NEB).