I'd say that it's homicide. Sometimes homicide is justified, and sometimes it's not. I think that defensive homicide is okay, and I think that's fairly uncontroversial.

And there's definitely a difference between directly killing someone and withholding extraordinary measures. What's extraordinary and what's ordinary will, of course, change over time with the progress of technology.

I'm not troubled by not tasking medical staffs with regulating hormones &c., as is required to keep brain-dead people alive. I'm rather more troubled by keeping them alive only to remove their organs piece by piece (but I find dead-donor transplantation morally problematic at best, anyway).