"Hamas sources in Gaza say 18 people suspected of collaborating with Israel have been executed."
I'm not sure why this surprises you, Isaac. The article you link to ends with
After the first 11 executions, Hamas warned that "the same punishment will be imposed soon on others". It added that "the current circumstances forced us to take such decisions",
suggesting a link between the executions and the killing of the three senior Hamas leaders.From the Hamas point of view, at war with Israel, such action - collaborating with the enemy - was presumably perceived as treason. Treason, in many Middle Eastern and Muslim countries is punishable by death. As it is in the USA (Article III, Section 3 - of the Constitution). In Europe the death penalty has been abolished.
Wikipedia:In the areas controlled by the Palestinian National Authority, it is treason to give assistance to Israeli troops without the authorization of the Palestinian Authority or to sell land to Jews (irrespective of nationality) and also non-Jewish Israeli citizens under the Palestinian Land Laws, as part of the PA's general policy of discouraging the expansion of Israeli settlements. Both crimes are capital offences subject to the death penalty [...] Likewise, in the Gaza Strip under the Hamas led government, any sort of cooperation or assistance to Israeli forces during military actions is also punishable by death.