The dice
You could say Billy died to his own hubris breaking into a restricted hospital ward guarded by 5 gunmen
But everyone in the party took the best precautions they could to avoid this happening once the situation turned as bad as it did.
Billy even overcame his anger and decided to escape instead of getting into a fight he was incredibly unlikely to win
But he rolled a 0.005175% chance to botch both of his efforts to escape
And the guards decided not to use their stunguns but instead to turn him into swiss cheese with their guns which 3 of them high-rolled with two 5 successes and a 6
an accumulated likelihood of 0.00000492%
Billy wanted to live but multiple gods wanted him extremely dead.
At least he had only had managed to do a single contract and helped make a move, this way it hurts less that he's dead because everyone is less invested in him.