- "You had your own men, soldiers, brothers walk into a trap?"
"And I'll carry that with me 'till the day I die. But 'till that day comes, I'm going to be a very, very wealthy man."
"You swore an oath, man!"
"Then Genesis day happened! ISIS, Taliban, Al Qaeda. I can take them all on, but what hope does the world have against men with magic? Or those meta-humans in Central City? How long before one of them manages to tear the world apart?"
- —John Diggle and J.G. Walker[src]
Walker tried to frame master sergeant John Diggle for the theft of a weapon of mass destruction and the murder of an enlisted soldier after MSG John's team took it from a terrorist group. His crimes were ultimately exposed, with Walker and his accomplices being kept under guard in a military prison until they can be court-martialed for their crimes.
Sometime after Genesis Day, Walker became the leader of John Diggle's unit while John himself was stationed in Eastern Europe. The unit mission was to recover a nuclear weapon which was stolen by terrorists, but after recovering it, Walker and other rogue soldiers betrayed John, framing him for the murder of a fellow soldier. He then fled the scene, with the nuke.
When John was arrested once again, after Prometheus tipped the federals, Walker wanted John to be transferred, so that he could be killed during the transfer, but Adrian Chase, who was serving as John's lawyer, managed to delay John's transfer by making John punch him.
After Walker crimes were exposed by Felicity Smoak with the help of Helix, Walker and his loyal men fled to Russia to sell the nuke to the Markovian terrorists, but they were ultimately stopped with the intervention of Team Arrow and the Bratva. He was then arrested by A.R.G.U.S., which placed him under custody so that he could be judged for his crimes.
Walker displayed a great lack of empathy, being capable of killing fellow soldiers just to frame John Diggle or selling nukes to terrorists for his own sake.
Walker is extremely arrogant, selfish, manipulative, greedy, power-hungry and immoral, as he broke his oath to the U.S. army when he stole weapon of mass destruction for money, he tried to justify this by stating that the world was coming to an end because of the existence of magic and meta-humans, he then killed an enlisted soldier in an attempt to frame John for his crimes, as he had no guilt or remorse for this vile act of his.
Although Walker was clever enough to use John's gun to frame him for murder, it wasn't to the point of covering his tracks very well, as the NSA were suspicious of John's so-called treason; as the latter doesn't have a record of insubordination or being discharge from the military.
Walker's arrogance is also to the point of being smug, whenever he believes his plans have succeeded, after he was exposed as the true culprit, he fled to Russia to sell the nuke to the Markovian terrorists (presumably to escape justice) but they were ultimately stopped with the intervention of Team Arrow. Walker then he tried to goad John into killing him so he would not do any jail time, by threatening the life of the latter's wife, Lyla Michaels and his son, John Diggle, Jr.; unfortunately, this completely backfired, as John saw right through Walker's attempt to avoid going to prison.
These negative traits of Walker's are what eventually led to his deserved/justified incarceration.