It's not insignificant that the finger in question is the middle one – the one you wave in the air like you just don't care. The hunter, being paid by oil companies to silence Little Bee and her sister, makes this explicit:
"White man been giving me this finger all my life. Today you can give it me to keep. Now cut off your middle finger mister and give it me." (4.294)
The hunter's comment refers to the colonization of his country (by England) but also probably to his own personal experience with white people. He sees Andrew as not only a symbol of all those other white guys, but as one such white man himself – one with money and attitude, who wants to take from Nigeria rather than give.
What the hunter seems to want from Andrew is acknowledgement of the tragedy faced by Nigeria and his role in it, however indirect. And so the hunter wants a sacrifice that is both symbolically potent and physically severe. He wants Andrew to understand that in this world, justice and salvation and safety have a high price – a price beyond money. It's all really ironic because the hunter is a brutal killer and a rapist. He's likely a victim-turned-victimizer who trades in a twisted and rough justice.