The Lord is depicted in this verse as the single agent of punishment upon the wicked, as seen in his word, on the day when I act.
The wrath of the Lord of hosts will consume the wicked on the day when God judges them, and they will be left as ashes upon the ground (Ezekiel 28:18). The faithful will tread upon these ashes. The point is that while the wicked are consumed, the faithful remnant will be spared (Malachi 3:17). They alone will survive the day of the Lord.
3 And you shall tread down the wicked, for they will be ashes under the soles of your feet, on the day when I act, says the LORD of hosts.