agreed, you can do the job without the need to relish in the fact that you are doing everything within your power to allow criminals to get away with all manner of offences.
Some people might be able to live with the fact that they let a guy go who raped a child, I myself couldnt.
Even worse that that, some people are overjoyed and feel pride in the fact that they allowed that to happen if it means it looks good on their CV.
You are overly sensitive to what Harley's job entails.
Your last remark is not unique to his job, and occurs in many if not all trades and professions.
Look at manufacturers who cut corners to speed up production, or the use of cheaper lesser reliable produce (not always the case, but you get the gist of it). We live in a prestige driven society, and cherning out numbers is the norm. As a result ethics and norms may become rather fluid for the better or worse, which are to be kept in check by institutions and regulations.
Now ofcourse it is important that Harley does his job well, if he has a poor history of cases in the sense of negligence or blatent bias not only will he not be hired he will also not be deemed reliable. This will negatively effect his job, and credibility of his profession as a whole. A part of Harley's job is do deal with otherwise morally difficult cases, if he was to be biased and not perform his job well, this wouldn't only reflect poorly of his work ethic, but also detract from the justice system as a whole. As has been mentioned several times prior.
To that end, I doubt Harley is very much invested in the 'personal' lives of his clients. But more so in generating an unbiased outcome, either in favor or against. Being able to detach and look at situations objectively is a conscious decision and required for such a job. Just like doctors need to distance themselves from clients as otherwise they wouldn't be able to function properly. This doesn't necessarily mean Harley doesn't have his own opinion about cases, simply that he's professional enough to not express it to others, which may affect an outcome.
Being a defense attorney isn't for everyone, just like factory work or being a CEO isn't for everyone.
Guaranteed though, that you'd be happy having someone like Harley at your side if you were in trouble, just to give a nod to the hipocrisy displayed here.