Can a Vanguard's Starfall kill a player at 100% HP? Maybe I'm missing something, but the Starfall is an ultimate ability which, if it directly hits an opponent, does a ton of damage. Shouldn't it be able to kill an opponent with full health or at least do enough damage to put it on par with the Enforcers homing missiles?
Based on the information gathered in this spreadsheet, Starfall does, at most, 300 damage (I assume this is if you land a direct hit with it). From the class HP values given here, Assassin has the lowest health pool, with exactly 300 HP.
Assuming this information is accurate, then it should be possible to score a kill on an Assassin with full health with Starfall. It's also probably worth noting that while Starfall will not kill units of the Vanguard, Gunslinger, Battle Medic, or Wraith classes from full health, it will do a very large amount of damage to them considering their max HP is only 325 for the first two and 350 for the second two. From full HP, a direct Starfall hit will definitely ravage their HP and with even a small amount of HP missing, it will kill them.