That depends on the law in each individual legal region (most probably country). For example, I believe Euthanasia is legal in some European countries.

Even the opposite can be true. Life saving medication may be illegal in one country but another provides it without restriction.

Kind of sums up my reply. Generally, if you know or there is enough suspicion the cure is going to harm or kill one and euthanasia is out of the picture then you are liable. That's why the whole process exists about medical trials. One nice window here is that you can in general experiment on yourself (people have and are doing that). Depends on the framework, but good point.

Anyways, my point here is you can not override law (or you shouldn't be able to).

Absolutety.

At least in the UK you can’t sign away your health and safety rights. Even if the job is dangerous the employer still has a duty of care.

Having said this there are some job roles that are excluded from specific parts of the legislation due to their unique nature (e.g. the Army)