“And baby I'll rule I'll rule I'll rule I'll rule.
Let me live that fantasy”

Lorde – Royals Lyrics on LyricsMode
In her line she is saying that she will rule whatever she is meaning about it.
And on the line "Let me live that fantasy.", you shouldn't stop her on living her own fantasy because it's hers.
Lorde means here that she is ruling her own fantasy on being a ruler in her own dreams.