"For God has put it in their hearts to EXECUTE HIS PURPOSE
Even in judgment, God's plan ALWAYS is executed. Mankind may believe that they control a 'KINGDOM' (Psalm 75:6-8), but, at best, they can really only make choices for themselves individually. This is particularly true in spiritual matters. God doesn't accept the notion of 'power of attorney' or 'class actions'.
Compare how Paul describes this as a 'deluding influence' (2 Thessalonians 2:1-11-12).
This phrase echoes verse 13.
The transfer is complete. In verse 13, just their power and authority were given. But now these 10 horns give their KINGDOM (singular, implying they were only governors in a larger KINGDOM) to the BEAST.
until the WORDS OF GOD should be fulfilled.
While these WORDS may be taken to refer only to those spoken here in the Revelation of Jesus Christ, in a broader sense, they are those spoken throughout eternity (that we know about through the Bible). They would include the creation (Genesis 1:3-5 through Genesis 3:22-24), and correlate with the redemption process (Revelation 5:1). Before the "sin curse" was placed on the earth, God spoke and saw that "it was GOOD" (physically and spiritually). God now is using evil leadership to separate evil from good or the tares from the wheat (Matthew 13:24-30). Once the tares are burned, the earth can be returned to its "GOOD" status. Interestingly, it doesn't matter what status in life a person has attained. Whether primitive native or sophisticated socialite, politician or common citizen, highly educated or intellectually challenged, one always struggles with spiritual/moral values. See the list in Revelation 21:8 of those persons who choose the wrong values.
From a prophetic viewpoint, this verse will confirm Revelation 14:8, Jeremiah 25:8-14, and Zechariah 14:1-9.