John Candy's death was kinda weird as I remember it. It seems like it was about 1994, and although his popularity was maybe on the wane, it seemed like it didn't hardly even make the news. Nowadays it would be all over the news, reported on constantly, speculated ( Matthew Perry comes to mind).
With John it was about 3 minutes into the evening newscast, John Candy dies at age 45, our next story is about a new computer feature you may use next decade called "Email".
I heard he died of a heart attack, but it always seemed a little fishy.
To some extent I feel like it's one of those "we saw that coming" sort of things. I doubt I was as involved in 1994 as I was 16 so I had other things going on, but Chris Farley would be a closer example to one that I paid a lot more attention to. When he died it was a "yeah, saw that coming" sort of thing. I'm sure for the older generation, John Belushi might have been similar.
I do suspect that a lot of comedians have serious demons. The "funny" is a defense mechanism for the real problems and they're just masking it.
Honestly, when I heard Robin Williams committed suicide... I wasn't that surprised. I think his entire career arc, and the personal stuff (reportedly a lot of drugs) IMHO were masking something much deeper. Although there was some speculation that perhaps there was something health-related involved (i.e. a chronic disease and he offed himself to escape what his near future would look like).
The non-comedy one that hit me REALLY hard was Anthony Bourdain. But again, you know that there were some deep-seated demons there too. I'm still not over it.
For both, it wasn't necessarily that they did it, but that they did it at a point later in life where it seemed like they had moved past the phase that you think it would have happened and on to other things.