Amalickiah continues to gain power through deceit and murder. When he brings his army to the king, the king suspects nothing. The king would not go out without guards, so they accompany him, but they were doubtless not on high alert. The murder occurs when Amalickiah’s servants had bowed down. That put them in a vulnerable position, which demonstrated their lack of harmful intent, at least according to custom.
When the king puts forth his hand to raise them, the first of Amalickiah’s servants kills the king by taking advantage of both surprise and the fact that the king’s arm was probably in a position where he could not easily defend himself against a strike.