To me a person seems "too far gone" when there are certain aspects that are simply vacant - like an absence of empathy, perpetual lying, a sense of satisfaction when cruel to others, etc. The person for whom the destruction of others is their goal for no reason other than a distorted sense of personal pleasure, is "too far gone". Such people will never choose to be anything other than what they are.
And for those who have lived sheltered lives, there are actually people like i have described. More than you might guess.