What would offend a Celt?

[Aodhan]

As there are no Celts today, I assume you're question is about pre-Roman Gaul or southern Germany. In that case, one Gaul to another may tell their enemy, that after he took his head, above the door he'd nail it to the shed, so his ugly face might scare away the crows. And for his body, if he could only hide the smell, he will chop it up and feed it to the dogs, if they'd deem it fit enough to eat?

What do you mean, there are still Celts in Scotland and Ireland, and most "English" people are in fact at least parts Celt, there are very few Roman bloodlines left in Britain. Did you perhaps mean Gauls?

And in answer to what would offend a Celt, the best thing you could possibly do is kill him, behead him, and then throw his head into a ditch and take the Torque, as a Roman "hero" did in some famous battle in CisAlpine Gaul, although I don't remember the exact battle.