The second, prosecutors say, is a bafflingly evil character nicknamed "The Terminator" who may well go down in history as the world's most prolific serial murderer.
After more than two years of delays, the trial of Onoprienko, 39, is due to open today in Ukraine, where he is believed to have killed at least 53 people, most in a three- month spree that caused chaos across the country.
The terror finally ended when he was arrested in Yavoriv, a small garrison town in western Ukraine, on Easter Sunday in 1996, after a manhunt that involved 2,000 police, thousands of national guards and more than 3,000 troops, some with armoured vehicles.
Although police say he has confessed to killing 52 people, investigators believe the figure may be higher, as there was a long gap between murders in which he travelled illegally in Hungary, Austria, Yugoslavia, Greece and Sweden. Unsolved murders through much of Europe were reopened after his arrest as detectives searched for evidence of his modus operandi.
His mental condition and motives for this orgy of killing are likely to loom large in the trial, which is in Zhytomyr, 80 miles west of the capital, Kiev.
Although he initially told investigators that he believed he was a superior human specimen chosen - possibly, he felt, by aliens - to kill his fellow human beings, detectives argue that he has no history of mental illness, and is both intelligent and cunning. He was, however, orphaned as a child; there has been speculation that, burning with resentment about his childhood, he was on a demented mission to wipe out as many families as possible.
It appears he particularly hated children; his younger victims were usually mutilated.
At least 40 of the murders occurred during the three-month rampage between December 1995 and March 1996, in which he caused such chaos that the government called out national guard units to protect several small villages in the rural area where the killings occurred. Entering houses before dawn, he slaughtered entire families and razed their homes.
After his capture, one investigator described him as the most perplexing person he had ever questioned.
"Usually, after a certain period of time, I come to a single conclusion about a suspect," Bogdan Teslya, a detective, told The Moscow Times in the month after Onoprienko's arrest. "But in Onoprienko's case, no matter how long I spend with him, I'm still left with two contradictory views of his personality.
"On the one hand, he is completely normal, intelligent and eloquent, obviously educated, a man who thinks before he says anything.
"But on the other hand, when he talks about the reasons for the killings, he completely loses his resemblance to a rational person."
The case has both horrified and intrigued Ukraine, not least because the killings coincided with a period in which Onoprienko was living blissfully with a woman whom he planned to marry.
During the three months in which he lived with Anna Kazakh, a 39-year- old hairdresser, and her two children, he is thought to have murdered 38 men, women and children.
He reportedly stole his victims' belongings - cassette recorders, rings, shoes - and bestowed them as gifts on his new family.
Although he frequently shot his victims, on one particularly gruesome spree he used another method: he went from room to room knifing a woman and her two sleeping children, one of whom was a three-month-old baby.
He did not like to waste bullets on those too weak to resist him, he later told police.