From page 16 of the FAQ:
What happens to the minions their General is defeated?
After a General is defeated, his minions remain on the board and will continue to spawn, taint locations, and overrun. The only change is that now there is a Slayer for that race that can defeat all minions of the slain General at their location by spending an action - no dice roll required.
In the night phase, do the minions still spread from darkness spread card even the General is defeated?
Yes, as stated above, the minions still spawn, overrun, and taint the land.