Well done BerkshireScouse
Thank you for being so very kind and considerate to an alternative life form.
Although the end outcome for your 'feral' mother cat was sad it was the humane decision to be taken in the circumstances (in my opinion) and you probably saved her from a slow and painfully extended death due to further viruses she may have contracted via continued sexual activity.
As a long term volunteer at an animal shelter I have seen (first hand) the agony of illnesses that uncontrolled breeding in feral cats and dogs causes - ie - feline immunodeficiency/leukaemia/parvovirus but to name a few.
As most shelters are solely reliant on charity funds they have to take a tough but fair decision when illness is detected to stop the spread to other animals in their care - the sick animal is put out of its current and future suffering via euthanasia.
TNR (trap-neuter-return) may not be an ideal solution with regard to managing the feral cat situation, but it is a least an option that will reduce the spread of controllable feline disease.
None of you expressing such distaste for TNR would want to contract any of the many potentially transmittable viruses or other problems that are attributed to feral life - toxoplasmosis/campylobacter/giardiasis/ringworm etc etc