A turquoise mask known as the Mask of Tlaloc, part of the Moctezuma: Aztec Ruler exhibition at the British Museum Photograph: Shaun Curry/AFP/Getty

The latest installment in the British Museum’s fascinating survey of significant rulers (past exhibitions have focused on Shah Abbas and the emperor Hadrian) focuses on Moctezuma. And the Mexica culture as communicated through this exhibition profoundly stretches the limits of cultural relativism, at least as far as I am concerned.

Clearly it is a trap to apply 21st-century western judgements to a civilisation such as that of the Mexica people in the 16th century. As Herodotus pointed out: “if one were to offer men to choose out of all the customs in the world such as seemed to them the best, they would examine the whole number, and end by preferring their own”.

And yet … I couldn’t help finding it profoundly chilling. First there is the sheer ugliness of the objects: the heavy basalt carvings so crudely made. Then there is the grotesqueness of imagery: it’s all about bloodletting and the sacrifice of humans. Here’s an extract from one gallery label: “Devout warriers would use these sharp thorns to pierce their legs, arms and earlobes and let their own blood as a mark of loyalty.” Ouch. Even the apparently most striking objects – the masks decorated with turquoise – are fantastically ugly. One of them, cheerily, uses an actual human skull as its template. Sometimes I wish I could time travel; but I think I’ll be giving 16th-century Lake Tetzcoco a miss.

Charlotte Higgins