Take a look on this sentence.
Today 71 years ago US dropped a nuclear bomb on Hiroshima killing over 1000000
Why in this sentence it uses killing, is it a noun and "Hiroshima killing over 1000000" is the noun phrase complement for this sentence?
A noun phrase complement is something that is linked to a noun using the verb be, for example:
Time is a great healer.
time is the subject and a great healer is the noun phrase complement.
Looking at your sentence, there is no be, so killing over 1000000 is not a noun phrase complement.
killing over 100000 is actually a participle clause. It is nonessential information, and should really be separated from the main clause with a comma:
Today 71 years ago US dropped a nuclear bomb on Hiroshima, killing over 1000000