Trump is the least competent opponent Iran could wish for.

The allies, in WWII had a similar choice. They decided not to kill Hitler. At that point he was making enough mistakes that his replacement would be sufficiently more competent to outweigh the benefits.

Right now, Iran is a quite limited war. Going after trump would massively escalate it. Iran doesn't want mushroom clouds over multiple cities.

Killing a head of state isn't actually effective. E.g. the ongoing 🇮🇷 war.