There is more to the story, not that it makes it right but it helps to paint a complete picture....
The following is from the Amnesty International Site, the original source of the information:
All three were killed within 24 hours of escaping from Gaza’s Central Prison. One of the brothers, Mahmoud, aged 24, reached the family home in
the middle of the afternoon of 28 December, but, an hour later, a group of gunmen came to the house and took him away......
As well as those accused of "collaboration" with the Israeli army, other detainees from Gaza Central Prison were found dead with gunshot wounds
hours after the prison was bombed by Israeli forces and the detainees fled. Among them was Nasser Mohammed Mhanna, a 34-year-old father of six, and a
commander of the al-Aqsa Martyrs’ Brigades, Fatah’s armed militia.
Nasser Mohammed Mhanna had been detained since August 2008 and was serving a two-year prison sentence on charges of participating in the attempted
assassination of Hamas’ deposed Prime Minister Isma’il Haniyeh.
Israel bombed a prison in Gaza from which people escaped, they were later killed and tortured by Hamas....
Edit to add: The posted source by the Op is incorrect
the attacks began shortly before Israel's 22-day military offensive targeting the Gaza Strip, and continued after a cease-fire took effect
The Began during the Israeli offensive not before.
Hamas’ deadly campaign took place during and after the Israeli military offensive
That is according to Amnesty Internationals site
[edit on 20-2-2009 by Obliterated]