If Israel or the U.S. or whoever were behind the assassination of an Iranian scientist they have only benefitted a tiny few people and put a lot more people at risk. Certainly it does not make Americans safer, it is unlikely it makes Israelis safer either. Iran will probably kill some people it would not have killed otherwise. The quietude with respect to Iran was unacceptable to more than a few political factions. The idea that Biden might reinvigorate some greater dialog with Iran was poisonous to some. Of course those who will be worse impacted by this are just people, people of little power. People in Iran, Israel, and the U.S.
The puppeteers pulling strings don’t care about people, especially little people. Iran does have to share some of the guilt here but this time pressed into a corner by whoever, they will lash back if only to prove they are not powerless.