Cornelius Slate

Captain  Cornelius Slate is a citizen and soldier of  Columbia, whose view on the military is based on both elaborate expectations and his real experiences. Once a proud supporter of  Zachary Hale Comstock, Slate became disillusioned due to Comstock's falsified war past, and was expelled from the Columbian military for calling it such. During the events of  BioShock Infinite, Slate leads a rabble of  "true" soldiers, who have taken over the  Hall of Heroes in  Soldier's Field.

History
A military man, Cornelius Slate served in the United States military and participated in the Battle of Wounded Knee, where he became acquainted with Booker DeWitt (whom he refers to as "Corporal DeWitt"). Some amount of time later, Slate became a follower of Zachary Comstock and a citizen of Columbia. In 1898, during the Spanish-American War, he led troops in the Battle of San Juan Hill. Three years later, Slate participated in the Boxer Rebellion on behalf of Columbia, where he lost his left eye and 30 comrades who fought under his command.

Soon after, Slate became disenchanted with Comstock, angered by the Prophet's false war past -- he claimed to have been present at the Battle of Wounded Knee, and the hero of the Boxer Rebellion, both claims Slate adamantly denied. When Slate confronted Comstock on the matter, he was stripped of his rank and branded a liar.

Filled with rage over Comstock's actions, Slate joined the Vox Populi and led a small team of soldiers to take over the Hall of Heroes, killing everyone inside, defacing the building and monuments within, and even changing outside signs to read, "Hall of Whores."

BioShock Infinite
After learning about Comstock's mechanical soldiers, Slate and his men desperately seek to die "as soldiers," gloriously in battle, and Slate considers Booker DeWitt the man for the job.

Booker, along with Elizabeth, travels to the Hall of Heroes to obtain the Shock Jockey Vigor. Slate leads Booker to two exhibits within the museum--the Wounded Knee Massacre and Boxer Rebellion exhibits--where Comstock features prominently as a war hero. Once Booker and Elizabeth reach the exhibits, they are attacked by vast squadrons of soldiers, who Slate sends to their "honorable" deaths.

Slate continues to test Booker, having him destroy Comstock's Motorized Patriots to prove his capabilities as a true soldier. Slate then reveals that he is in possession of the Shock Jockey, and creates a series of electric traps while Booker battles a final squadron. With the battle finished, Booker pursues a weakened Slate to obtain the Shock Jockey. (On an interesting note, before he sends in the last squadron, he has a health bar. Even after you attack him, he'll still set up his Shock Jockey traps, send in the squadron, and run away.)

Slate then gives Booker one final order, handing him a pistol and telling him to "finish it." If Booker follows through, Slate's last remarks are about Booker not changing and still being a true soldier. If spared, Slate angrily yells at Booker, calling him a "Tin Man." In the second scenario, Slate is subsequently captured by Columbian authorities and imprisoned at the Good Time Club, where he is found tortured into a catatonic state. Once again, Booker has two choices: kill him or leave him behind. If Booker kills him, Elizabeth will respond with, "I guess that's what he wanted."

In a parallel universe, where the Vox Populi have taken over Columbia, Booker joins forces with Slate at the Hall of Heroes. Together, they tear it down, at the cost of their own lives. They are declared martyrs of the revolution, with Booker appearing on Vox propaganda posters.

Personality
Heavily disillusioned by Comstock's betrayal, Slate has become a fanatic, desperate for him and his men to die honorably in a final blaze of glory. He therefore refuses to allow his men to die at the hands of the Motorized Patriots, and instead forces Booker to fight them. His abuse of Shock Jockey results in the Vigor's crystals growing from his skull like tumors, impeding his ability to make rational decisions and exacerbating his delusions. He refers to Comstock's soldiers and Motorized Patriots as "Tin Men" to distinguish them from his concept of the ideal, true soldier. Overall, Slate is portrayed as a battle-loving warrior who finds solace and purpose through violence.Slate holds very high expectations of those within the military due to his own experience as a soldier. He respects individuals who have proven their merit, like Booker Dewitt and Vivian Monroe. Consequently, he is infuriated that Comstock, a civilian with no military record, at least to Slate's knowledge, receives "undue" glory, when he receives none. In point of fact, some aspects of Comstock's military history, though romanticized, are true. Comstock did fight at Wounded Knee, but at the time Slate knew him as Booker DeWitt.

Trivia

 * Slate has some small fragments of Shock Jockey crystals on the right side of his head, which might be the side effect of the said Vigor. This may be due to Slate being originally part of a rejected Vigor Junkie concept for Splicer-equivalent enemies, mentioned in The Art of BioShock Infinite.