Possible meanings are 1) the beast rules twice: first as one of the seven kings, and then as the eighth king or 2) the beast belongs to that group of seven kings because he is like them.
The certainty of what will happen in the future is spoken of as if the beast were going to it. AT: "it will certainly be destroyed" or "God will surely destroy it" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-activepassive]])
