Leonard Killer was the youngest son of William Killer and Sarah Jane Lamb, who were married on 30 November 1876 at the parish church in Middleton by Wirksworth. Sarah was a widow and brought two sons to the marriage.

The young couple lived in Middleton by Wirksworth after they married, and by 1891, when the census return was made, they had eight children at home in addition to Sarah Jane’s two boys. William was a stone merchant and quarryman, and the family had a domestic servant living in to support the household.

Leonard was born on 3 January 1894 and taken to the parish church for baptism on 11 February. On 11 January 1897, he started attending Middleton by Wirksworth National School. When the census was taken in 1901, the family was living at 21 Freehold Street, Derby. William’s stepsons had left home and two of the children had died. William gave his occupation as limestone polisher, and five of the remaining children were working, leaving just Leonard still at school.

It appears that the family moved house again to 56 Agard Street, Derby, and Leonard was enrolled in St John’s School on 16 October 1905. The family still lived on Agard Street in 1911 when the next census return was made; there were just two children still at home, 17-year-old Leonard, who was working as a stove and grate fitter at Russell and Co. His father and older sister Marianne both worked as clerks at J. P. Doherty & Co.

When war broke out, Leonard was living in Duffield. It appears that he enlisted in the territorials in Derby towards the end of 1912 or early 1913. He joined the Sherwood Foresters (Nottinghamshire & Derbyshire Regiment) and was posted to B Company, 1st/5th Battalion. He was initially given the service number 1765.

This battalion of the Territorial Force was in Derby when war broke out and was mobilised to Harpenden, moving on to Braintree in November 1914 before embarking for France towards the end of February 1915.

Within three days of arriving in France, the men were in the trenches under instruction, and so began what must have seemed a relentless round of being in the front line, in reserve, and resting. That year the battalion fought at Hooge when the German troops made a liquid fire attack, and in the attack at Hohenzollern Redoubt.

In July 1916, the battalion fought in the diversionary attack on Gommecourt, aiming to draw some of the enemy reserve troops away from the main Somme offensive taking place further south. Many of the territorial men were given new service numbers when the system was reorganised in 1917, and Leonard was issued with number 29193.

In 1917, the men were involved in the operations on the Ancre, the Occupation of Gommecourt and the attack on Rettemoy Graben before the German retreat to the Hindenburg Line, the attack on Lievin, and the Battle of Hill 70. Leonard served almost three years on the battlefield before he spent his 24th birthday in heavy snow in trenches near St Elie.

At the beginning of March 1918, the battalion was near Erny St Julien, preparing for battle once more. On 6 March, after two days of travelling by train, bus, and marching, the men arrived at La Preol for a week of specialist training and bayonet training drill. There followed six days in the trenches, when enemy artillery was very active, before moving into Brigade Reserve near Beuvry.

On the night of 21/22 March, the enemy raided the Brigade Front; several of the dugouts occupied by the men of B Company were blown in. The men were relieved the next day.

On 28 March, the battalion went into the front line in the Emile Sector. About 10.40 p.m. on the evening of 29 March, the enemy began a bombardment along the front and support line. At 11.25 p.m., the bombardment appeared to be intensifying; A Company fired the S.O.S. Rocket and sent a message by wire to Battalion Headquarters. There followed a further 23 minutes of bombardment. During that time, 14 Germans were seen advancing near the wire — the men opened fire with rifles and Lewis guns.

Several men in the battalion were killed or wounded, and their trench was also damaged. This resulted in there not being enough men immediately available to follow the enemy as they withdrew. The report in the battalion war diary states: "Our men showed a fine spirit, and thanks to their steadiness, the enemy failed to enter our trenches or obtain identification." One of the casualties on that day was Private Leonard Killer. He was buried in the Aix-Noulette Communal Cemetery Extension.

For his service to his country, Leonard earned the 1914–1915 Star, the British War Medal, and the Victory Medal. His War Gratuity Payment of £17 and accrued pay were sent to his mother, Sarah.

After the war, when the Commonwealth War Graves Commission erected a headstone for his grave, Leonard’s father paid 3/2d for the inscription "Well beloved" to be added. William and Sarah were living at Victoria Villas, 182 Uttoxeter Road, Derby by then.

Leonard is also named on the Middleton by Wirksworth memorial; a tablet of Hopton stone on a background of black marble, surmounted by carved crossed rifles and a laurel wreath. The inscription on the memorial reads:
"To the glory of God and in memory of the following parishioners who gave their lives for their country in the Great War. Their names shall be held in everlasting remembrance 1914–1918."

Leonard’s father died in 1926, and his mother moved to Osmaston Road before she died in 1937. Both were buried in the Old Cemetery, Uttoxeter New Road, Derby.

His brother Ernest served in the Dragoon Guards throughout the war, earning the 1914 Star, the British War Medal, and the Victory Medal. He died in Derby in 1976.