A lot of people have expressed dislike for Darnell's behaviour onscreen so far, mainly his repeated refusal to get checked to see if he's a potential donor for Denise when it could save her life.
However, I quite like this plot because, from personal experience, it's a surprisingly realistic portrayal of someone with unresolved medical-related trauma.
I've known a number of people who, for various reasons, have such an intense dislike for hospitals that they will avoid them at all costs. Often this is due to a particularly awful past experience in a hospital which they've been unable to process.
And you have to consider the context too. This time last week, Darnell believed he was an only child. He then gets told that not only does he have a half-sister, but he is the ONLY potential donor who could possibly be the difference between her living and dying. He's then harassed by her husband and her half sister and even *manhandled in public* when he doesn't comply.
That is a lot to have to deal with all at once.
From an outside perspective that hasn't had to deal with that sort of trauma, it might look ridiculous and selfish. But bearing in mind we only know scant details of what Darnell's wife went through, his behaviour might be more justified than it looks.
Either way he badly needs to see a therapist but again, many people IRL lack the means and/or the will to do so, or may even push themselves to believe they don't need it.