Take vengeance on the Midianites for what they did to the people of Israel

Yahweh was punishing the Midianites for convincing the people of Israel to worship idols.

you will die and be gathered to your people

These two phrases mean basically the same thing. This is a polite way to emphasize that it is time for Moses to die and for his spirit to go to the place where his ancestors are. (See: rc://en/ta/man/jit/figs-parallelism and rc://en/ta/man/jit/figs-euphemism)