She knew giving birth to a child in mortal realm meant draining her divine power? she told her son he needed her…
It's a love tribulation and she was willing to have a child with him despite the consequences. She probably thought he didn't want to because he doesn't care about her enough to bother.
He couldn't cross because she flooded the path. She wanted him to know that the Wei blocked them.
Maybe in the novel? They mentioned that by morning the reservoir would be filled and they cant cross. Plus i dont think she would kill people when there's another way.