We have for a long time sold military equipment (or given it as aid) to nations that are despotic. Would you argue we are to blame for their actions? Should we be punished if a dictator uses American weapons to suborn the people?
I completely agree with you that Iran has much to answer for. The question is, is this one of the things. I certainly didn't blame the US for Iran using US equipment against Iraq back in the day. I don't blame us for MBS killing the journalist though certainly our military equipment guarantees MBS stays in power. Though we should seriously reconsider our support.
Here is an article from 8 years ago of ISIS growing in Gaza. I don't think it is likely, but it is possible that Hamas has been infiltrated by ISIS and thus isn't completely the group Iran has been backing (ISIS and Iran are bitter enemies). Again, not likely, but I've heard a lot of people, including IDF leadership, say this attack was much more like an ISIS attack than a Hamas attack. So I don't think we can rule out that Hamas has splintered and part of it is under ISIS management.
A video statement issued by ISIS channels in Syria addressed the "tyrants of Hamas" and vowed to uproot the Palestinian organization.
www.nbcnews.com
Again, I think it is unlikely the Mossad missed such an event. But given this matches what we saw in ISIS' advance across Iraq, it isn't impossible.
Going back, I was very supportive of George W. Bush as he resisted pressure to lash out immediately and bomb everyone back to the stone age after 9/11. He waited, we gathered our facts, and we concluded beyond doubt exactly who was responsible. It was all perfect up until he decided to add Iraq into the equation. Biden has the exact same obligation here. Other solutions may be improbable, but improbable ≠ impossible. You are a lawyer, you know there are degrees of culpability. Iran selling weapons to a known danger is bad, but it isn't as bad as working with the known danger directly to conduct genocide. We aren't going to strike Iran over the former. It is an open question over the latter.