Ask Lemmygrad

They either see a chance to destabilize the government enough so that the reformists completely capitulate (like they've wanted to do). Or Trump is trying to force some lesser negotiation concession and make it look like a win like Maduro.

It depends on whether or not the US believes its own propaganda about Iran's stability/capabilities. I see a real possibility that if they assassinated Khamenei and other high ranking members of the IRGC, Washington may think that would propel the reformists into absolute power and they then concede the missle program, which the IRGC and Guardian council have fiercely opposed negotiations on.

However that's only if Trump and the current clique in power genuinely believes in supremacy of the US.

The more likely thing to happen if Khamenei is assassinated I'd that the IRGC decides that the country is in an emergency scenario and begins liquidating reformists if they think they undermine Iranian sovereignty. They'd likely have popular support after a US operation as people are really sick of the reformists reproachment rhetoric with the west.

Figures like Rohani who before used to proudly discuss these concepts are now opportunistically changing their tune to something more similar to what the IRGC has been saying for a decade. That signals to me that Iranian public opinion is now very clearly mobilized against any negotiation with the US, which the government is still fruitlessly engaging in (Astagfurallah).