It's Mature in the US. Frankly, MANY families in the US wouldn't be phased by the killing of tiefling children. Parents let their kids watch Return of the King at very young ages with decapitated heads and eyeballs popping out. BG3 is tame compared to some of the stuff parents here are letting their kids watch these days.
Mature isn’t the highest rating a game can get though. Return of the King is an odd comparison - those were PG-13 films. It has a lot of violence but how it is presented and against whom is rather important. I was very squeamish at the time, and had no issues watching LotR trilogy. I found original Star Wars trilogy more violent then LotR.
It is a curious limitation though - it has so much “mature” content I find it difficult to believe Larian would worry about the game being inappropriate for young audience. There specifically created a limitation, though, so there must be a reason for it.
