Could you forgive your child if they committed cold-blooded murder?

We forgive our children everything. it’s called ‘unconditional love’. The dark drama Born To Kill (C4) gives this natural parental instinct a sinister, psychotic twist — and asks, what if some things are unforgivable?

Sam and Chrissy are a couple of teens whose erratic behaviour seems about to drive their doting single parents to despair. Played by Jack rowan and Lara Peake, he is a charming loner and liar who is bottling up the urge to commit murder — and she gets her kicks setting fire to the school.

But Sam’s mum (romola garai) thinks her boy is a saint. He cooks for her, and drops in at the hospital where she works as a nurse, to read to the elderly patients. What she doesn’t see is that he’s a narcissist, spending hours in his bedroom taking videos of himself.

Chrissy’s dad (Daniel Mays) is under no illusions. He knows his daughter is a troublemaker. But he blames himself, for not being more supportive when her mother died.

This is an uncomfortable set-up, and for the first 45 minutes we were kept in doubt — were we being too harsh on these kids? Perhaps they were just sensitive souls, struggling with teenage angst.

Then Sam smothered an old chap in his hospital bed and stole his watch, and we realised this was more than just a bad case of moody hormones.

Television has tackled this issue before, in one of the great eastenders storylines. Launderette manager Dot Cotton (June Brown) turned herself inside out, trying to explain away the murderous antics of her drug- dealing son, Nasty Nick (John Altman).

She was ready to overlook everything, even his cold-blooded plan to kill her — but the more she forgave, the worse she punished herself.

in both eastenders and Born To Kill, these conflicting parental impulses are most dramatic when the crimes of the child are less excessive. it is unsettling to watch a drama, asking yourself: ‘Could i forgive those lies?’

But it’s much harder to imagine how you’d feel if your son or daughter committed murder, unless that appalling catastrophe has struck your family in real life. The more Born To Kill strays into melodrama, the more it will lose its tension.

The unimaginable tragedy that consumed mother- of- three rhian after her one-year-old son died five years ago overshadowed Mind Over Marathon (BBC1), the first of a two-part documentary that explores whether vigorous physical exercise can allay the symptoms of grief and mental illness.

rhian’s boy george died within hours after contracting pnuemonia. She was powerless to help him, and has been paralysed by self- doubt and recrimination ever since — quite unjustly, since anyone can see she is an outstanding mother.

Her pain was redoubled five days after george’s death, when her despairing husband walked out of the house and took his own life. Now rhian cannot hear a car pull up outside without being filled with dread that the police have returned with yet more bad news.

Nick Knowles gave her space to talk about her pain, and gently encouraged her as she joined nine other people with different types of depression, anxiety and psychological illness, in training for the London Marathon.

Prince William made a guest appearance, with the promise that he, Kate and Harry will be along next week to cheer the runners through their paces.

The only niggle was the wretched ‘sad piano’ soundtrack that is apparently obligatory on all programmes about mental health. But we’ll forgive that.