Two hours later, the 32-year-old barrister was dead, leaving friends and family shocked as to how a young man who had led such a charmed life could have died so violently.
He was a respected barrister, married to an equally talented colleague and together they had recently moved into their £2.2m Georgian flat in one of London's most prestigious neighbourhoods. Yet something drove him to start firing a shotgun out of his window and to continue shooting.
"It has been a very bad day. We're still trying to come to terms with it," said his mother, Rosemary Saunders, 62.
"He had seemed completely normal only hours before the incident," his father, Rodney, 64, said. "It is an absolute mystery."
Some details have emerged of the five-hour stand-off between the lone gunman and armed police. Earlier, Mr Saunders had left work at one of the country's leading chambers for family lawyers, based in the Temple district of central London. He was in an apparently good mood but spent much of the day drinking, reports suggested. Upon his return home, he argued with his wife, Elizabeth Clarke, 40, who neighbours saw fleeing the flat in Markham Square, Chelsea, in tears.
At about 5pm, showing little emotion, he began taking random shots at surrounding buildings. A neighbour, Jane Winkworth, said she screamed at him to stop. Then she ran into her flat and called the police. "When I screamed at him ... he didn't say anything. It was as though he wasn't listening," she said. "All he kept saying to the police was 'I can't hear you'. He was mumbling stuff like a drunk person would but he wasn't shouting or being aggressive apart from when he was shooting. The shots were very loud. They were absolutely terrifying."
Leslie Hummel, whose home backs on to the scene, told how she looked on in horror as he blew out one of his windows. He then fired a shotgun round at one of the first police officers to arrive, who grabbed his sidearm and returned fire.
Evidence of the gun battle is obvious, including windows smashed by gun shots and a door battered open by police. Local people spoke of homes being peppered with shotgun pellets as armed police flooded the area and the confrontation developed. Marksmen took up vantage points on balconies and at windows in surrounding properties.
Shop workers and customers in the adjoining King's Road were forced to lock themselves in, while neighbours were told to shut themselves in their bathrooms as Mr Saunders and police exchanged fire several times over the next five hours. Negotiators tried to talk to Mr Saunders, but he made no demands, only replying: "I can't hear you, mate."
At about 7pm, he threw an empty white box into the garden. In black pen, he had written the message to his wife. The last volley of shots was exchanged at about 9.10pm, before the final development 20 minutes later. Witnesses saw green flashes when members of Scotland Yard's CO19 firearms unit stormed the flat using stun grenades. Minutes later emergency medical staff were ushered into the cordoned-off area where the semi-naked gunman lay injured. Unconfirmed sources said that Mr Saunders had been shot from a distance before the police entered the second-floor flat.
Yesterday, his father, Rodney, from Cheshire, said the family were trying to understand what had happened. "We are not aware of anything in his work life or private life which might have made him react like this," he said. "It is bad enough to have a bereavement in the family but this is exceptional by any standards. You could understand it if he was a terrorist, but Mark was not a terrorist."
Described as a "very talented person", Mr Saunders was educated at King's School in Macclesfield before studying law at Christ Church, Oxford. Called to the Bar in 1998, he joined the QEB chambers where he met his wife, Elizabeth Clarke. The pair married 18 months ago and moved into their new Chelsea home eight months later. The pair were described as an ideal couple by friends.
Mr Saunders joined the Honourable Artillery Company, which prides itself on being one of the Territorial Army's most prestigious regiments. He left in 2002 without being deployed on operations abroad.
A specialist in divorce work and domestic violence cases, he was described as a "popular, gutsy and polished" lawyer who had attracted a "volley of praise" and had a great future ahead of him. His wife, also an Oxford graduate, is equally respected in the profession and said to be "an absolute superstar", an articulate barrister who specialises in divorce cases.
A spokeswoman at QEB said: "He was a very valued member of the chambers and we are shocked and horrified by what has happened." One described Mr Saunders as a "funny, charming, clever man" who would be deeply missed.
Despite their apparent devotion towards one another, it is clear something went badly wrong between Mr Saunders and his wife. Last night, reports emerged that alcohol was a fixture in their relationship and Mr Saunders was described as a "hard drinker" by unnamed friends.
"Mark was great fun, and he loved Elizabeth dearly, but he had a terrible weakness for alcohol," one said. "He would drink red wine and whisky at all times of the day and it was destroying their marriage."
It was also claimed by one neighbour that the pair were trying for a baby.
The Independent Police Complaints Commission said yesterday it was still awaiting the results of a post-mortem examination and it was too early to say if he had been killed by police bullets. A legally held firearm was recovered from the address.
