Mister-Baseball.com, a site about baseball in Europe, has details on Halman's professional baseball career:
Gregory Halman was born on August 26, 1987 in Haarlem, Netherlands. After making his debut in the Dutch Hoofdklasse at the age of 16 he was signed as a non-drafted amateur free agent in 2004 by the Seattle Mariners. He quickly moved up through the Mariners farm system and reached the big leagues in 2010. He appeared in 44 MLB games for Seattle since then.
Halman hit .230/.256/.345 with two home runs, six RBI and five stolen bases over 91 plate appearances in 35 games for the Mariners in 2011. He had shown some power in the minor leagues, hitting 33 home runs for Triple-A Tacoma in 2010.
According to the MSNBC.com report, Halman's brother has been arrested in connection with the stabbing:
"A 24-year-old died this morning in a stabbing and we have arrested the 22-year-old brother of the victim," a Rotterdam police spokesman told Reuters. Officials declined to give the suspect's name, in line with Dutch privacy rules.