How is that an excuse? You send weapons to a genocidal regime, knowing they're currently doing genocide, you are guilty. Your ulterior motives doesn't matter.
"Ok Mr Judge I gave that arsenic to that serial poisoner but he told me he wanted to defend himself against rats. I knew he poisoned a bunch of people but again not my business".
Because, as a country and a people Israel does, in fact, have the right to exist and defend themselves. They are going about it 110% the wrong way, but no American president or congress has the balls to tell Israel "Sorry, kid, you're on your own, good luck!"
Until that happens, or some other country intervenes directly in Israel, it will only get worse.
The killing of civilians, especially able bodied males is rationalized in preemptive attack, as they are accused of plotting against the perpetrators. The perpetrator may exterminate witnesses and relatives of the victims.