Upvote:4
The Wikipedia article you linked to has the explanation. Bernadotte was killed by the Lehi group, also known as the Stern Gang.
The group regarded Bernadotte as a stooge of the British and their Arab allies, and therefore as a serious threat to the emerging state of Israel. Most immediately, a truce was in force and Lehi feared that the Israeli leadership would agree to Bernadotte's peace proposals, which they considered disastrous.
After the Holocaust, some Zionists decided that their only hope of safety lay in the use of force against anything they considered a threat. Bernadotte's past actions didn't count for much if they considered him a threat later on.