For all we know, the more likely scenario is that Charlie, like a sizeable percentage of his doctor peers, was burnt out, tired, and depressed, did not really have an overwhelming (some might say "healthy") desire to survive (in fact, perhaps quite the opposite), and saw the cancer as a non-undignified quick "way out".
Doctors (and medical professionals more generally) rank among the highest in occupational risk of mental health disease, especially for things like addiction, alcoholism, generalised anxiety, ptsd, depression and suicide.
I have no objection regarding the choice he made, but let's not glorify it as the "natural" thing to do either. This narrative is harmful to people who "do" desire to survive but are scared, which may then prevent them from making a dispassionate decision regarding their care.