Perhaps the disabled recognize just how easily a homicide could be masked as assisted suicide?
Yes..that's why there are concerns even when there's support. Anyone would be stupid or crazy not to have concerns. I have concerns about RKBA, but support it. I have concerns about people drinking and driving, but I'm not against either.
It's called sanity. Just because someone is disabled doesn't mean he's insane. But in a rational society, we address concerns. There are many concerns as the law exists, such as people making botched suicide attempts, or having to kill themselves before they become too infirm, rather than waiting and having their remaining time.