Alright, let me clarify.
He is just a knesset member; he does not represent Israel's military or economic value, so he is a low-value target srategically. Furthermore, if he remains alive, he will continue to make genocidal remarks, which can help Iran gain more political correctness and international support. His existence can also uningry Iranian people and secure their support for the war.
If you kill him with a missile, you will waste a missile and gain nothing more than some venting of anger from people. Some Iranians might be satisfied by this and turn to supporting a ceasefire and negotiations. Moreover, you have not damaged Israel's core interests, and killing a legislator will not affect teasefire and negotiations. Therefore, choosing this target will undoubtedly lead other countries to doubt Iran's resolve to resist aggression.