Human sacrifice as per Christianity isn’t a bad thing, given it was consensual and done in order to fulfil an important purpose (saving people from their sins and Hell). Meanwhile, the Aztecs sacrificed people who didn’t want to be sacrificed.

It’s like calling a soldier whose throws themself on a grenade to save their squad (human sacrifice) “dark and awful”, or saying the same about a group of soldiers in a POW camp where they the enemy picks one of them to be tortured and killed (and one soldier volunteers to do so to save the other soldiers in their group).

Also relating back to the OP’s question, vastly more people believe that Christianity is true as opposed to the Aztec religion being true, so they think the Aztec sacrificed were useless (killing for no reason as the Aztec religion is false) while the Christian sacrifice did indeed serve a purpose. But then again, even if the Aztec sacrificed were necessary, the main difference would still be that Jesus willingly sacrificed himself but the Aztecs’ captives didn’t.