It is (still) a free country Bobby, and so they can do anything they like...but...
the risk is not only to themselves, it is to everybody else that they meet. To those who cannot be vaccinated, to vaccinated people who get infected (and may not know it) and take it home to their vulnerable others. To children at school, and schools have to close as too many children are infected, to NHS staff who have to isolate, and hence put even more pressure on the NHS through staff shortages.
And your example of "killing themselves" involves an awful lot of other people, for a long time (Ambulance, NHS staff, support staff, grieving relatives, expensive medicine, preventing other people using the NHS. ).
In my view (and it is my view, and I am entitled to say it). It is selfish