HISTORY
If you ain't guilty, then why you runnin'?
Hint: Hovering your mouse over [QUOTE]"Blah blah blah..."s reveals what Sgt. Coyle has to say in-game or in the comic...
Sources:
Childhood
Leland grew up the small town of Blackwell, Oklahoma, likely on a cattle farm. As a child, he was a perpretator of sexual abuse1 and violence against animals (typical early signs of mental illness and/or psycopathy in children), landing him in a military academy.
His relationship with his parents is shaky at best. Coyle has described his mother as "not right in the head" after an event where she forced him into a bath [QUOTE]"Wouldn't believe me. Knew from birth I was marked to die by lightning. Momma forced me into the bath, twisted my vittles and shoved me in the water even though she knew the electricity could sneak up them pipes and cook me like a suckling pig. Bitch never was right in the head.". He also states that she "didn't raise no nancies", and that Mother Gooseberry "reminds [him] of [his] sweet mother", indicating there isn't all bad blood between them (or, he was being sarcastic with the latter statement). There's also a new line introduced in Project Boneyard: [QUOTE]"Lotta people kiss their mother on the mouth.". I'm unsure what to make of it right now, but I figured it should be mentioned.>
Coyle's father is/was a WW2 veteran who lost his foot "during Hurtgen Woods" (likely the Battle of Hürtgen Forest), and seems to be emotionally distant from his wife and son, described in how he neglected to kill a cow when it was caught ablaze after being struck by lightning, instead deciding to crack open a beer and watch [QUOTE]"Saw a cow struck by lightning once. Kraut black angus; caught fire and screame like a child. Momma told Daddy to kill it but he just opened himself a beer and sat down to watch it galavant and caper. It ain't always a humdrum time in the country.".
The latter incident, along with Coyle identifying a specific kind of beef cow ("kraut black angus"), and how "it ain't always a humdrum time in the country", leads me to believe that he did grow up on a cattle farm, though there is no explicit evidence saying so.
As these issues went unchecked, his teenage years naturally transitioned into criminal activity, further reinforced by recruitment into his local Ku Klux Klan chapter.
Military
At age 19, Leland married his first wife, then joined the military after she died. Her official cause of death was 'falling down the stairs', but as he joined the military in order to avoid investigation, it's implied he's the one that killed her [QUOTE]"Reminds me why I killed all those wives of mine." (along with the rest of his wives in the future).
He servered two years in the Pacific Theater honorably, with three confirmed enemy kills and two suspicious American deaths within his company. In the Muroff Collection comic, it's illustrated that he's kept trophies of his kills, including ears and teeth (though, these don't necessarily have to be from his time served in the military.) It's implied that he has fought in (at least) the Battle of Okinawa in 1945 [QUOTE]"Ain't seen an innocent man since Okinawa.".
Police Dept.
Post-military, Coyle returns to Oklahoma and resumes Klan activity, naturally then finding work in the local police department (assumingly, in Blackwell.) On police payroll since 1947, Leland found success in gaining profits from exploitation of prison labor, extortion, and civil forfeiture. He became very well-known both in-department and around town, with multiple commendations.
When Leland married his second wife, he began to extort her family, who then later died in an electric fire. His wife fleed to Chicago and later died from what was officially states as 'natural causes', and her body was later discovered during an Easter Egg hunt.
After being promoted to Sergeant, he marries a third and presumably final time. His last wife died by 'suicide' with multiple gunshot wounds in her head. His standing in the police department made it very easy for him to avoid responsibility in his wives' deaths. His wive's deaths would later become a catalyst for his extreme paranoia, further explored in the Personality section.>
Murkoff
Sergeant Coyle was investigated by Murkoff's senior intelligence officer, Clyde Perry, under the pre-tense of bribary in order to find recruits for Project LATHE. Upon interview with Clyde at a bar sometime early Feb. 1956, Leland states that he's been aware of how he's been followed [QUOTE]"If you think I haven't seen you tailing me since Tuesday... you must think I'm stone blind and pig-ignorant.".
Within minutes, Leland attacks Clyde after realizing his intention. After the attack, Clyde went to recover in Wichita, Kansas with broken fingers, bruising, and bloodied urine. During recovery, he greaty reccomends Sgt. Coyle for Project LATHE.
Inside of Murkoff Corporation's Sinyala Facility, Coyle takes the role of a 'Prime Asset'. It's unknown how exactly he got into the facility, as the parts in-between his interview and actual recruitment into Project LATHE is unclear. As a Prime Asset, Coyle takes sadistic pleasure in ruling his trials with an iron fist.
1 Many people seem to have misinterpreted this line from the "Murkoff Collections" in-game document as him being sexually assaulted. The words are "Anecdotal evidence of abuse of animals and sexual assault as a child, leading to military academy education." To me at least, this clearly implies that Leland himself was the perpretator of both sexual abuse and animal abuse, being what landed him in the military academy.
However, there is loose evidence to support the theory that Leland is a victim of CSA, such as describing how his mother "twisted [his] vittles" ("vittles" being slang for a young chubby boy's breasts), and displaying (potentially) hypersexuality at a very young age.