A year as an auxiliary nurse in a hospital taught me many lessons.

Apart from learning never to get trapped in a toilet cubical with a farmer the size of a Hereford bull, the most startling was that behind the seemingly blank or comatose expressions of geriatric patients, there was still a spark of life.

These vulnerable men and women, many of whom had dementia or had suffered a stroke, were eager to communicate, and be listened to. They often showed humour too, as if the ability to laugh is one of the last vestiges of personality to retreat beneath the ailments of old age.

I have been reminded of this lately, when visiting a local care home, where rows of old eyes lift at each new arrival, only to sink back into torpor when they see a stranger.

You could be fooled into thinking these residents were barely conscious, condemned to living a life the young and fit would consider unbearable. "Put me out of my misery before you put me in a care home," is the usual response of the middle-aged, as they eye encroaching old age. And yet, when these elderly individuals start to talk, they do not seem unhappy or suicidal, merely tired, brave, and a little lonely or bored.

Their image comes to mind with every new twist in the debate over Lord Falconer's private member's bill on assisted dying, which has its second reading on Friday. In recent days, Archbishop Justin Welby has spoken against the proposals, while former archbishops George Carey and Desmond Tutu have declared themselves in favour, citing the rights of everyone to dignity and Christian compassion, which includes being helped to end intolerable distress.

Although Margo Macdonald did not cite religious reasons, her attempt to introduce an assisted dying bill at Holyrood won her widespread respect. Since being diagnosed with Parkinson's, she said, she could envisage a time when "in the event of losing my dignity or being faced with the prospect of a painful or protracted death, I should have the right to choose to curtail my own, and my family's, suffering."

And, although her proposals were rejected, a poll suggested 69 per cent of the country agreed with her. One can be sure that if Lord Falconer's bill were passed, it would influence legislation here. As moral quandaries go, this is one of the hardest a society can face.

Anyone who has seen a person in desperate pain or torment would wish their condition eased or ended, and the emotional agony of their carers and families with it. The good intentions behind the idea of assisted death are unquestionable and, if each case of assisted suicide could be judged on its merits, I might be in favour. What is hard to accept, however, is blanket legislation that, while offering mercy to a small number, might also become an invisible weapon in the hands of the unscrupulous.

Lest we forget, Alzheimer's is probably the most prevalent terminal illness in the land. Having seen the neglect the elderly and demented often endure, parked by their families in care homes and hospitals as if they were broken-down old bangers, I am in no doubt pressure, both overt and subtle, would be brought to bear on some to request assisted suicide and thereby make life easier for their relatives.

Assessing whether such applicants' wishes were their own, or imposed, would require astonishing perspicacity and patience. An unwanted "burden" on a family, sitting on a dwindling inheritance or living in a hostile household or unvisited home, might easily be bullied or cajoled into agreeing to depart. By such time, they might feel so unwanted that their decision would stem from genuine mental anguish, even if not prompted by their physical condition.

Supporters of assisted dying will claim if it is not made legal, the responsibility for helping those who want to die will fall on their carers, putting them in legal jeopardy. All of us would hope never to be in such a situation, but if we were, this terrible decision would surely be taken in the hope of leniency, and the comfort of having ended misery, even if one is left to contemplate it from behind bars.

Far more worrying is the idea that under new legislation, cold-hearted killers would sleep easy, as if a thorn had been removed from their side.