I don’t think GRRM thinks particularly highly of the concept of “honour”, or people who use it as an excuse to do either terrible things, or do nothing when terrible things are being done. However, I also don’t believe we are meant to see the situation as entirely black and white. Jaime ‘s actions were taken under extreme circumstances, and very few people would criticise the, if they knew the truth. But in the days and years leading up to that final straw, many respected, presumably honourable, experienced older men stood by and did nothing as terrible things happened, because they valued their loyalty to the institution of kingship, and perhaps the stability of the realm, more highly than they did the welfare of the individuals being affected by Aerys’ madness.

This makes me wonder if Barristan’s very name is drawn from a real world institution where people are asked to do something difficult things in the name of the greater good.

In many common law systems, a 'barrister' is a type of lawyer who works independently and who usually specialises in litigation or advocacy - ie. appearance work in courts, and especially in higher courts. The profession is prestigious and still attracts many of the best lawyers and has a near monopoly on appointment as judges.

So, what is the relevance of this to Barristan the Bold?

Well, one of the more interesting, and misunderstood, requirements of practice as a barrister is the "cab rank rule". This rule requires that, much like a taxi driver (cabby) sitting at a rank, barristers must accept work on a first-come, first-served basis. This is the case even if the nature of the case or the conduct, opinions or beliefs of the prospective client are unacceptable to the barrister, or to the public, or even if the barrister has any belief or opinion (as opposed to knowledge) of the guilt or innocence of that person.

The rule is not absolute, and there are some exceptions I don’t need to get into. But the rule absolutely prohibits a barrister turning away a client because they disapprove of what she has done, or don't like the cut of his jib. Accused murderer or child molester? Tobacco company? Slum landlord? If you’re a barrister, and the person's cause falls into your area of practice, and they can pay, you generally must take their case, and exercise your best efforts to represent them, subject only to your duties to the Court (including your duty not to mislead it). The rationale for this is, put simply, that, as a barrister, your primary duty is to the justice system as a whole. By making yourself available at large, you ensure everyone can get representation, even those who society deems undeserving of it.

Of course, you can‘t take the analogy very far - a barrister absolutely can’t hide behind client privilege to aid and abet a crime, and if it all becomes too much he can put aside his wig and gown at any time, while a kingsguard is stuck for life. The concept of duty to the courts and to justice is also a far more sophisticated and nuanced concept than the Westerosi King. But the similarities between the names are very similar, and even though the USA has abolished the distinction between barristers and solicitors, there is a very good chance that GRRM is aware of the historic distinction (he has probably seen an episode or two of Rumpole of the Bailey). I think George is probably inviting us to ask questions about what the the role the kingsguard plays, the nature of oaths and duty, and when loyalty to the institution of the “crown” should be set aside because the man wearing it is not worthy of it.