According to Josephus, they didn't kill any Romans. "Now for the Romans, they expected that they should be fought in the morning, when, accordingly, they put on their armor, and laid bridges of planks upon their ladders from their banks, to make an assault upon the fortress, which they did; but saw nobody as an enemy, but a terrible solitude on every side, with a fire within the place, as well as a perfect silence. So they were at a loss to guess at what had happened."
On the other hand, Josephus also tells us that the Sicarii--the group occupying Masada--did venture out before the Romans got there and massacred 700 Jews in a nearby village on Passover. "As for such as could not run away, being women and children, they slew of them above seven hundred. Afterward, when they had carried every thing out of their houses, and had seized upon all the fruits that were in a flourishing condition, they brought them into Masada. And indeed these men laid all the villages that were about the fortress waste, and made the whole country desolate."
Appears the Zealots killed 6,000 Romans in one battle in 66 AD.
That appears to be what pissed off the Romans.