What is safe to destroy?

[Ludens]

But the Romans sacrificed a Gaul and a Greek after Cannae. Enemies, in other words. Not their own people. And then only in the direst of emergencies. Unlike the Carthaginians, who routinely sacrificed children of their own blood. In Roman eyes, that was a key difference.

Gladiatorial games, again, a different matter, because gladiators were just slaves, not Roman citizens. Slaves in the arena were mostly captured enemy warriors, who deserved nothing better. So that wasn't hypocritical, in Roman eyes.

So what the Romans were opposed to was religious sacrifice of citizens rather than humans?

I can understand their reasoning, but that does not change the fact that they did commit human sacrifice. Unless, of course, one argues that enemies/slaves do not count as humans, which simply leads to another form of hypocrisy.