How is it bad? Imagine a world where instead of sending hundreds of thousands young men to die, countries would just launch targeted attacks on the head of enemy's state.

Would you say the same if iran killed trump?

I cannot confirm or deny that I would say the same thing

They're civilians, for one.

Are they? In most countries the head of the state is also army's commander-in-chief.

Yes, but these are traditionally not treated as actual soldiers, but civilians commanders-in-chief-in-name-only.

The powerful people that set the rules have made it unacceptable to kill powerful people.

Powerless people? Into the meat grinder!

If more dictators fear for their lives: good.

If he didn't want to be assassinated maybe he shouldn't have killed all those people.

We already have a bad president.

Quite the opposite - if they know they are risking their lives they would be more reasonable.

It's the stated reason why the United States has an impeachment process. So that they have a process for removing undesirable heads of state without resorting to assassination.

True but other countries don't have an equivalent process.

Most have, but the decition is made by the local politicians instead of foreing nuclear powers. It's actually very similar to the impeachment.

Sorry I phrased that poorly. I meant countries have no means to impeach a foreign head of state so impeachment only serves as a power check if the citizenry disapproves.

Not that I think heads of state fearing for their lives from airstrikes is necessarily good, being able to act with impunity is certainly bad.

The US has a constitution as well? It seems pretty worthless these days since nobody is willing to enforce it..