The Chicago Police Department Changed Jay Halsted Too Fast
In Jay Halstead's final episode, he becomes a completely different character. And there is no explanation for his change.

Jay Halstead makes a change in the final episode of the Chicago Police Department, but too quickly. After Jesse Lee Soffer asked to leave, the writers needed a way to write him off the show, so they created a storyline that had him leave Chicago. While it's a good thing the writers chose not to kill Alvin Olinski (Elias Koteas) as he did with him, it didn't stay true to the character fans know and love.
Soffer first appeared in The Great Chicago Fire. He then went on to star in the Chicago Police Department. He was a former military sniper who came home to care for his mother when she had cancer. He's been a fan favorite in two of his favorite relationships, Erin Lindsey (Sophia Bush) and Hailey Upton (Tracy Spiridakos). Surprisingly, Soffer announced he was leaving the show after nearly 10 seasons of the big role.
Jay Was Always A Boy Scout

Jay Halstead never liked the way Hank Voight (Jason Beghe) did things. He did mention his dislike of Voigt's methods to his colleagues in the early seasons, but didn't confront him until later. Still, Jay's mistrust was always palpable. As he gets to know Voight, he asks for more information flinched. He hated Erin for standing by Voight after he killed his son's killer. Jay didn't want Erin to go down with Voight. Jay has had nine seasons of disapproving of Voight's ways, so it doesn't make sense for him to change his personality and the way he works so quickly.
From the beginning, Antonio Dawson (Jon Seda) was the counter-attacker when Voight crossed the line -- Voight didn't always listen. After Dawson left the Chicago Police Department in Season 6, Halsted became the team's counter-runner. Having someone against Voight balances the show when Voight goes rogue. It makes sense for Halstead to take over the role after Dawson leaves.
Jay Made Into A Voight In Training, And It Didn't Ring True

In Halstead's final scene, he tells Voight that he wants to be like Voight, but Voight tells him he doesn't and shouldn't try. While he admits he's not like Voight, it's surprising that after fighting Voight for so many years, he suddenly wants to be his boss. Halstead is an excellent cop who never crosses any line until Hayley Upton gets involved in Voight's mess.
in season On the 10th, Halstead began spending a lot of time with Voight, both on and off work. He stood by Voight for every call he made, even lying to Hayley about working late while he and Voight had a drink. It's as if the writers were going to cast him as Voight's protégé. It's odd to see Halstead do a U-turn from someone who disagrees with how Voight works.
Regrettably, the Chicago Police Department removed Jay Halstead from the show. Jay would never leave his wife and move away. Instead, he'll try to make things work. While this might have been a great character development for Hayley, it ruined Jay's role on the show.