Various logs found throughout the game reveal that he was a devout and likely a very high-level Unitologist, as evidenced by his hand-selection by the church to lead the extraction of the Red Marker. He was one of many who became increasingly delusional over the Marker's influence, which he demanded be taken immediately to the Church, as was his mission all along. Apparently, he made rash decisions; such as denying Hydroponics to share their food surplus, and quarantining the entire colony on Aegis VII (ironically, to prevent the colonist's insanity from spreading to the Ishimura). Through a log made by Matthius, it is revealed that he believed the Church planned on publicly revealing the Marker once it was in their possession (though, given the clandestine nature of the Church, it is unknown whether this was the Church's true intentions). Also, as alleged in a log made by the Chief Comms Officer, after attempting to call for help, it appears that Matthius had overloaded the communications array in an effort to prevent any off-ship communication that could jeopardize the mission.
His last log was that once the marker is retreive, the governmetn won't be able to cover it up.
As events progress and Matthius becomes more and more erratic, Dr. Terrence Kyne declares him unfit for active duty and relieves him from command, ordering the Ishimura's security guards to restrain him. As Matthius struggled, the sedative needle was lodged in his eye by Dr. Kyne, killing him; it is unknown whether this was an accident or a deliberate act by Dr. Kyne. A text log, containing an autopsy of Matthius, suggests that the needle went through the eye, through the skull and into the frontal lobe, killing him instantly. The coroner suggests that it must have been someone else, as the strength of the stab couldn't possibly be of himself, as a person would have too much self-restraint to do it; the circumstances which led to his death are somewhat unclear.
Matthius' body was taken to the ship's morgue's examination room, where his body lay while the Necromorphs began to overtake the ship. To access the Bridge's computer, the crew of the USG Kellion needed Matthius' RIG. Eventually, when Isaac Clarke came to retrieve it from the corpse of the former captain, an Infector lept upon it, thereby resurrecting Matthius as an Enhanced Slasher (the first type of this enemy encountered in the game). Isaac promptly killed the creature, putting him down for good and taking his RIG.