Short answers, pulled from the story.
Sviatopolk I earned the sobriquet "the Accursed" after allegedly ordering the murders of three of his brothers, Boris, Gleb, and Sviatoslav, to eliminate rival claimants to the Kievan throne. His actual responsibility for the killings has been disputed by historians, with some foreign sources suggesting Yaroslav, not Sviatopolk, was responsible.
Sviatopolk I was Grand Prince of Kiev from 1015 to 1019, though his reign was interrupted. He first seized Kiev in 1015, was defeated by Yaroslav near Lubech in 1016, retook the city in 1018 with Polish military support, and was finally defeated at the Alta River before dying in July 1019.
Sviatopolk I died in July 1019 while fleeing toward Poland after his final defeat at the Alta River by Yaroslav. It has also been suggested he may have been killed by a descendant of a figure identified in sources as Valuk Conqueror.
Bolesław I the Brave was Sviatopolk's father-in-law and his primary military ally. Sviatopolk had conspired to go to war against Vladimir with Bolesław's support early in his life. In 1018, Bolesław led an army into Rus' that defeated Yaroslav and restored Sviatopolk to Kiev, before withdrawing to Poland and seizing the Cherven towns on the way home.
A warrior buried at Bodzia Cemetery in Poland, dated to around 1010-1020 AD, carried the Y-DNA subclade I-S2077 and possessed the richest burial in the cemetery. Strontium analysis of his tooth enamel showed he was not a local. Researchers believe he was closely related to Sviatopolk, arrived in Poland during the events of 1018, and may have been Sviatopolk himself.
The chronicle of Thietmar of Merseburg, written by a contemporary who died in 1018, can be read as suggesting Sviatopolk fled Kiev to Poland immediately after his father's death, before the murders occurred. A Norse saga, Eymund's saga, places blame for a brother's murder on Yaroslav rather than Sviatopolk. Some historians argue Sviatopolk actually ascended the throne after Boris's assassination and sought to punish Boris's killers.