The short answer to your question is that in border states the views on secession were not homogenous, and many people from states that stayed in the Union supported the Confederacy, and many people from states that seceded were Unionists (most notably, of course, West Virginia actually left Virginia and was admitted to the Union on June 20, 1863).
We’re going to address why someone from Maryland (Booth’s home state) would have been a Confederate supporter, and then go into specifically why Booth himself was so fanatical (at least as much as we know why he acted the way he did, or that the way he acted was based in any sort of rational thought).
In 1860, there were around 90,000 slaves in Maryland, and the free black population was roughly the same size. One in six white families in Maryland owned at least one slave—the reason that these families would have supported the Confederacy (in general) is clear. Moreover, many whites who did not own slaves were against abolition, just as poor whites in Southern states who did not own slaves were against abolition.
In 1860, Maryland was a critical strategic point for the Union. Washington, D.C. was (and is) border by Maryland on three sides and Virginia on the fourth. Virginia had already seceded, Maryland’s departure from the Union would have put the capital in great danger. On April 19, in what we know now as the Baltimore Riot of 1861, the 6th MA militia regiment, on their way to D.C., passed through Baltimore. (At the time, Baltimore did not have a direct railroad through the city, so the Union soldiers had to detrain, walk roughly 10 blocks, and then board the train to D.C.)
The 6th MA was under strict orders not to initiate gunfire, but after members of the mob fired pistols into the marching soldiers, they returned fire. At the end of the day, the soldiers had killed 12 Baltimoreans and wounded an unknown amount of others, and four members of the 6th MA had been killed by the mob, along with 36 wounded.
The Maryland state song to this day, Maryland, My Maryland, written by James Ryder Randall, references the riot: “The despot’s heel is on thy shore, Maryland! / His torch is at thy temple door, Maryland! Avenge the patriotic gore/ That flecked the streets of Baltimore/ And be the battle queen of yore / Maryland! My Maryland!”
On April 29, the day that the Maryland state legislature voted, 53-13, against secession, voted against opening rail lines to Union soldiers and asked Lincoln to remove federal troops from the state, both of which Lincoln ignored. In response to the former directive, the Governor of Maryland, in support of the legislature, allegedly ordered the destruction of numerous railroad bridges within the state.
John Merryman, a lieutenant in a Maryland militia, was arrested on May 25 for his connection with the bridge-burning. Chief Justice Roger Taney, a wealthy, slave-owning Marylander who had previously presided over the now-infamous Dred Scott decision, issued a writ of habeas corpus, requiring that George Cadwalader, the general commanding Fort McHenry, where Merryman was being held, either bring Merryman to a court or release him. Cadwalader refused, and Lincoln supported Cadwalader.
Taney wrote his opinion, stating that it was an unconstitutional use of presidential power, and then, as he was also one of the two federal judges for Maryland, refused to try Merryman at any point during the war, determining that he would not receive a fair trial in wartime Maryland.
Lincoln famously said, on July 4, 1861, “Are all the laws, but one [habeas corpus], to go unexecuted, and the government itself go to pieces, lest that one is violated?” Booth himself, like many Marylanders, supported the claim that the suspension of habeas corpus was unconstitutional.
Now, let’s dive into the life of John Wilkes Booth. John Wilkes Booth was born near Bel Air, Maryland on May 10, 1838. His family owned a large farm, around 150 acres, and his family, just like the state of Maryland, was divided on the morality of slavery. His father was said by some (including Edwin, Booth’s older brother, who himself was a staunch Unionist) to have hated slavery, but the Booth farm was at times worked by slaves rented from neighbors.
We know what John himself would come to think of slavery, from a letter he wrote which was published during the manhunt in the New York Times in 1865—“And looking upon African Slavery from the same stand-point held by the noble framers of our constitution, I for one, have ever considered it one of the greatest blessings (both for themselves and us,) that God has ever bestowed upon a favored nation.”
Junius Brutus Booth, the family patriarch, was an actor, and so were the three brothers John Wilkes, Edwin, and Junius Jr. John Wilkes had a flair for the dramatic, as many actors do, and his favorite role to play was Brutus: killer of the dictator Caesar. Critics and contemporaries described Booth as more vigorous and energetic on stage than his brother Edwin, who was typically more subdued.
The three brothers performed together once, on November 25, 1864, when John Wilkes played Mark Antony and Edwin upstaged John Wilkes in playing Brutus in Shakespeare’s Julius Caesar.
John Wilkes Booth was strongly anti-abolitionist. He attended John Brown’s hanging in 1859 and was greatly satisfied with what had befallen Brown. According to his sister, Asia, John Wilkes smuggled quinine into the South on theater tours, when it was difficult to purchase due to the Union blockades. He was arrested at least twice for “treasonous” speech, once in Albany in 1861, and once in St. Louis in 1863—it seems on both occasions he expressed verbal support for the Confederacy.
In 1864, he was involved in an abortive plot to kidnap Lincoln, which morphed after Lee’s surrender at Appomattox into the assassination. By 1864, John Wilkes and Edwin were estranged, and Asia remembered John Wilkes as a ranting, raving man, who reserved a special hatred for Lincoln. On April 11, 1865, Lincoln gave a speech from the White House window, in which he suggested extending the vote to African-Americans, and Booth flew into a rage.
He killed Lincoln three days later, on April 14, Good Friday, 1865. After he had shot Lincoln and jumped onto the stage, he infamously shouted “Sic semper tyrannis,” the words Brutus is said to have said after he killed Lincoln.
One of the difficulties in answering this question is that the fanaticism John Wilkes Booth felt for the Confederate cause was very likely irrational. We hope we’ve explained why it’s not necessarily contradictory that Booth was from Maryland but supported secession. However, beyond that, the devotion to the Southern cause required to kill Lincoln was not one that at that point in the war could have been based on a logical train of thought.
We know that Booth was ashamed at not having fought in the war—he was in his early 20s during it, but he had promised his mother he would not enlist. He later wrote to her, about that decision, ”I have begun to deem myself a coward and to despise my own existence.”
Some historians have argued that he felt the need to upstage his brother Edwin; he certainly must have seen himself as a parallel to Brutus, who was his favorite character to play and the origin of his most infamous quote. However, when all is said and done, Wilkes Booth certainly possessed an irrational hatred for Lincoln and an irrational fervor for the Southern cause, or he was just attempting to dramatically write himself into the history books.