The scariest thing I have been a part of in FIRST was testing shooter prototypes for Stronghold. From the terrifying-looking "double barrel" spring-puncher to the time we were manually drawing back the puncher and the rope snapped (this resulted in the puncher rod hitting the ceiling hard enough to tear it). The scariest robot-combat moment I have seen is this:
https://m.youtube.com/watch?v=stSRTk_CAlg
(Another angle)
https://m.youtube.com/watch?v=avyj5kQ8v0s
The scariest ROBOT I have seen, however, has to be Hellachopper (this thing was disqualified from Battlebots season 2 because it was deemed "too dangerous". It's weapons are either spinning hammers that can reach 8' in diameter while traveling at the speed of sound or "beast mode", which has a 6' diameter, shown in the video below.
https://m.youtube.com/watch?v=_vph7wZEFAc