Baby Face Nelson

Rocca engineered Gillis's release from Joliet prison specifically to hire him as a hitman to kill a labor organizer. However, Gillis refused this assignment, preferring instead to work with Rocca's gang on robberies. This refusal set Gillis on his own criminal path rather than becoming Rocca's personal assassin.

The exact motivation for refusing isn't explicitly stated, but Gillis clearly preferred the more profitable and less politically charged life of a robber alongside Rocca's gang. He was willing to commit crimes for money but drew a line at what appears to be a targeted political assassination, showing he had some boundaries despite his violent nature.

Mob boss Rocca deliberately planted a gun in Gillis's hotel room to frame him for the labor organizer's murder. When police burst in, they discovered this evidence, confirming Rocca's betrayal. This treachery motivated Gillis to seek revenge and eliminate Rocca and his men.

Earlier, Rocca had hired Gillis for a job he refused, setting up the betrayal that follows.

After killing Rocca and his henchmen, Gillis chose to take Sue's surname 'Nelson' as his alias. This was both a way to honor his partner who helped him escape the police raid and revenge his ambush, and it gave him a new identity as he embarked on his criminal rampage.

After being wounded during a pharmacy holdup, Gillis sought medical help from Doc Saunders, a physician who secretly treated America's most wanted criminals, including John Dillinger. While recovering and being treated by Doc, Gillis entered Dillinger's criminal circle and earned the nickname 'Baby Face Nelson.'

Nelson killed Doc Saunders in a fit of anger, demonstrating his escalating volatility and complete lack of loyalty. Doc had helped treat him and facilitated his connection to Dillinger, but Nelson's rage led him to murder the very doctor who aided him, showing his descent into uncontrollable violence.

Nelson pointed a rifle sight at children on city streets, terrifying Sue and demonstrating his complete moral depravity. This act marked a turning point in their relationship, showing Sue the true depths of Nelson's cruelty and foreshadowing his ultimate fate, he had become so desensitized to violence that he would threaten innocents without hesitation.

Wounded and cornered by police, Nelson staggered into a graveyard where he pleaded with Sue to end his suffering. He then taunted her, daring her to pull the trigger. Sue finally shot him, granting his request for a mercy killing rather than letting him be captured or die slowly from his wounds.

After John Dillinger was shot and killed in Chicago, the mantle of America's most notorious criminal passed to Nelson. His relentless killing spree, including murders of law enforcement and innocent motorists, and his high-profile criminal activities made him the FBI's new Public Enemy No. 1, surpassing even Dillinger's notoriety.

During a pharmacy robbery, Gillis was shot and wounded, forcing him to seek medical treatment. This injury led him to Doc Saunders, who treated criminals including John Dillinger. Without this wound, Gillis would never have entered Dillinger's orbit and never become 'Baby Face Nelson.'