I have trodden the wine-press alone - I, Yahweh, have indeed trod the wine-press of my wrath, and I have done it alone (compare the notes at Isaiah 34:5-6). The idea here is, that he had completely destroyed his foes in Idumea, and had done it by a great slaughter.
For I will tread - Or rather, I trod them. It refers to what he had done; or what was then past.
And their blood shall be sprinkled - Or rather, their blood was sprinkled. The word used here (נצח nētsach) does not commonly mean blood; but splendor, glory, purity, truth, perpetuity, eternity. Gesenius derives the word, as used here, from an Arabic word meaning to sprinkle, to scatter; and hence, the juice or liquor of the grape as it is sprinkled or spirted from grapes when trodden. There is no doubt here that it refers to blood - though with the idea of its being spirted out by treading down a foe.
And I will stain all my raiment - I have stained all my raiment - referring to the fact that the slaughter was extensive and entire. On the extent of the slaughter, see the notes at Isaiah 34:6-7, Isaiah 34:9-10.