"You have neither listened nor inclined your ears to hear, although the Lord persistently sent to you all his servants and prophets" -Jeremiah 25:4
“Therefore thus says the Lord of hosts: Because you have not obeyed my words, 9 behold, I will send for all the tribes of the north, declares the Lord, and for Nebuchadnezzar the king of Babylon, my servant, and I will bring them against this land and its inhabitants, and against all these surrounding nations. I will devote them to destruction, and make them a horror, a hissing, and an everlasting desolation" -Jeremiah 25:8,9
It is very worthwhile to notice that in these verses from Jeremiah 25, the LORD declares that the prophets (who warned Judah of the coming judgement) and Nebuchadnezzar (who took the people into exile) are called servants of the LORD. These are two very different servants. On the one hand, the prophets were called out by God to go and serve Him (see Jeremiah 1:4-10 for God's setting apart of Jeremiah) On the other hand, Nebuchadnezzar was the king of Babylon who received no such special call to serve the LORD. As F.B. Huey, Jr. remarks in his commentary, "The designation of Nebuchadnezzar as 'servant' is a reminder of God's sovereignty. He can use a person to accomplish his purposes whether that person is intentionally or unwittingly serving him (cf. Rom 9:17)."