I think that there are two dates of the start of WW2:
1.9.1939 is the Western/European start date
7.7.1937 is the East Asian/Chinese start date
The ending of WW2 has also different end dates:
8.5.1945 is the Western/European date (Russians consider it to be 9.5.1945 because of its signing time in Moscow´s time)
2.9.1945 is the East Asian date, but is nowadays recognized as the final end of WW2. The US, Chinese, Soviet, British, Commonwealth countries and some European countries were in war with Japan up to that date and that is still a lot of globe in war.
There are also different dates to be argued. Britain and France declared war to Germany on 3.9.1939 (not on 1.9.1939, but German-Polish war 1939 is widely recognized as part of WW2)
From the East Asian/Chinese point the Chinese-Japanese war should be considered as part of WW2 right from the start of 1937. Or was it suddenly another war called WW2 since 7.12.1941, when USA and other Western countries became allies of China? One could argue that Sino-Japanese war became suddenly a part of WW2 in Dec. 1941, but Chinese do not share that view and they have reason to do so.
"First American citizens killed in WW2" means thus both civilians or volunteer fighters KIA before the Pearl Harbor Attack 7.12.1941. If You take 1.9.1939 as start date, then the victims on board of Athenia are definitively the first US victims of WW2. If You take 7.7.1937 as a start date, my guess is that the two crewmen of USS Patay and American captain of one oiltanker were the first US citizens killed in Sino-Japanese war. The assault and sinking of USS Patay was also the first Japanese attack against US Navy vessel (which had a visible flag and other US markings).
The choice is yours. Most Western/European historians consider 1.9.1939 as a start date of WW2. East Asian/Chinese and also few Western historians consider 7.7.1937 as a start date.
Just to add: Arthur Chin is considered in USA (at least by
American Airpower Heritage Museum in
Midland,
Texas) as a first American ace pilot of WW2. He participated in Chinese-Japanese War in late 1930´s and achieved his air victories long before the Dec. 1941.