There is no proof that this man was actually planning an attack - just an intelligence report which may or may not be accurate. It is certain that the people injured for being near him weren't involved in any plot. But that doesn't matter - Israel has the right (according to you) to attack them. If Israel was wrong, it doesn't matter because they were only Palestinians so they have no rights.
Stabbing reported in Kiryat Gat in southern Israel (Haaretz)
from the article: IDF: Gaza strike killed terrorist planning attack on Egypt border