If they slaughtered Cortez and five other Cortezes that came afterwards, it wouldn't matter. Guns, germs, steel, horses, and the seething hatred of the Aztecs by the people they brutality oppressed predetermined their fate.
Montezuma invited the Conquistadors into the city and they even held him as a hostage for months before any fighting broke out. Those alliances took a while to establish and the germs only came later into the campaign when they started sending ships back for more troops during a Caribbean smallpox outbreak.
I recommend reading Conquest by Hugh Thomas. The whole story is fascinating and very nuanced - it took several expeditions to even get that far and Cortez was possibly the luckiest man alive in the 16th century.