Short answers, pulled from the story.
Didius Julianus became emperor by winning an auction held by the Praetorian Guard in March 193 AD. After the guard murdered Emperor Pertinax, they offered the throne to the highest bidder. Julianus outbid his rival Sulpicianus by offering 25,000 sesterces per soldier, and the guard proclaimed him emperor.
Didius Julianus reigned for only 66 days, from the 28th of March to the 2nd of June 193 AD. He was sentenced to death by the Senate after being abandoned by the Praetorian Guard, and was killed by a soldier in the palace.
According to the contemporary Roman historian Cassius Dio, Didius Julianus' last words were: "But what evil have I done? Whom have I killed?" He was killed in the palace on the 2nd of June 193 AD.
Before becoming emperor, Didius Julianus had a long military and political career. He governed provinces including Dalmatia and Germania Inferior, defeated the Germanic tribes the Chauci and the Chatti, and was appointed to the consulship in 175 alongside Pertinax. He was raised by Domitia Calvilla, the mother of Emperor Marcus Aurelius.
After Didius Julianus was executed, Septimius Severus became emperor. Severus dismissed the Praetorian Guard and executed the soldiers who had killed Pertinax. The Senate also passed a damnatio memoriae condemning Julianus and his legacy.
Didius Julianus reversed reforms made by his predecessor Pertinax by devaluing the Roman denarius. Pertinax had raised the silver content of the denarius to 87 percent; Julianus reduced it to 75 percent, nearly restoring the debased level of the Commodus era.