Legally, not justifiable. Even though sailing at an unusually high speed, she was not in the declared blockade zone, she wasn't zig-zagging (was she?), wasn't armed and didn't resist, wasn't travelling in convoy or under escort and wasn't darkened at night. Still very much a "neutral" merchant vessel, despite Lend-Lease, the US "security zone" extended out to 22 deg W and USN escorting convoys as far east as Iceland.
At that time, the U-Boats were forbidden to attack American vessels and they were instructed to avoid any incidents with the United States. Your success was 3 days after the U-203 unsuccessfully attacked the USS TEXAS (BB-35), mistaken for a RN dreadnaught, which resulted in a further order to the U-Boat force to "positively identify" any vessel as hostile before making an attack...practically impossible to do without being detected.
But then, it's a game and when boredom sets in, just "sink 'em all!"
