base defiler

Amsterdam gets caught while trying to kill Bill, who is a gang leader. Bill says:

I want you all to meet the son of Priest Vallon. I took him under my wing and see how I'm repaid? Saves my life one day so he can kill me the next
like a sneak-thief instead of fighting like a man. A base defiler, unworthy of a noble name.

(Gangs of New York)

What is the definition for "base" here? Thanks for your help.

Base means amoral, unprincipled, ignoble, inferior (it's an adjective in this context).

There is also an archaic definition as being of low social class - this is also partly the meaning being used here (Bill refers to "unworthy of a noble name"). The idea, of course, being that people of low social class exhibit amoral, unprincipled behaviour.