Actually, in personal self-defense you absolutely do! Because the knife you don't see is the one that is most likely to kill you. Part of defense against knife attacks is knowing there is a knife present before one sticks you.
That, Shusha, is hilarious!
So you go scouring the streets looking for someone with a knife so that you can practice your self defense!
Classic!
Your argument is just getting more and more ridiculous. You are equating a massive riot of tens of thousands of people with thousands approaching the fence in a mix of militant operatives and civilians incited to be human shields. Scouring the crowd for weapons and threats is EXACTLY what Israel is doing. Entirely successfully. It is eliminating those threats because they are threats. The primary purpose of those actions are to eliminate the threats and prevent them from crossing into Israel to harm Israelis. The secondary purpose is to avoid a massive breach of tens of thousands over the fence which would result in a much more difficult defense situation and a much larger number of Gazan dead. It is a perfectly reasonable and effective military strategy. It is also a moral military strategy because those being deliberately killed are militant operatives who pose a threat. (No, its not perfect system wherein no civilians have been killed. But it is an extraordinarily accurate given the circumstances of military operatives being disguised as and embedded with crowds of innocent people).
You are trying to make an argument, ridiculously, that proportionality is a tit-for-tat death-for-death. If one Israeli dies, then somehow it is "permissible" for Israel to "defend" itself by killing one Gazan. Or, you are trying to make the equally ridiculous argument that there is no threat posed by those militants taking part in the riot using the cover of human shields. Neither argument is a good one.