The word "him" here also refers to the king of Assyria. But it does not mean God is only sending the king; he is sending with him the army of Assyria. Alternate translation: "I send the army of Assyria ... I order them" (See: [[rc://en/ta/man/translate/figs-synecdoche]])
Yahweh speaks of his wrath as if it were more liquid than a container could hold; "the people" are trying to carry this container, but it is heavy, and Yahweh keeps pouring liquid in even after it starts to spill out. Alternate translation: "at whom I continue to be angry even after I have punished them" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Possible meanings are 1) Yahweh compares the army of Assyria attacking Israel to people stomping through mud who do not care what happens to the mud. Alternate translation: "trample them until they become like mud" or 2) the people are stomping on other people so they are lying in the mud and unable to rise. This is a metaphor for completely defeating them. Alternate translation: "completely defeat them" (See: [[rc://en/ta/man/translate/figs-simile]] and [[rc://en/ta/man/translate/figs-metaphor]])