Iran to attack before they respond?

Mostly because the degree to which military action is provocative and destabilizing depends (in part) on whether it is an attack or a counter-attack.

Israel and Iran are not in a hot war. So launching a military strike would be a massive escalation in tensions in the area....but less so if it is in response to being attacked first. Israel felt that the value of killing Haniyeh was worth ratcheting up the tension somewhat, and all parties recognize that it will lead to some response from Iran (though probably not a massive response). But it's in Israel's best interests (and probably Iran's) for things to not get out of hand.

So Israel makes it clear that when Iran responds they'll respond again in turn. That's a useful message to send, since they want Iran's response to be small enough that they can possibly just ignore it or retaliate with some non-military response. But it's not in their interests to just go out and attack Iran.