What did revolutionaries accomplish by assassinating alexander ii?

They have stopped his pursuit of proposing additional parliamentary reforms to counter the rise of nascent revolutionary and anarchistic movements and eventually preventing his decision to emancipate the serfs. Russia’s military inferiority, weak infrastructure and a backward economy based on serfdom, was exposed, confirmed for the new tsar the need to modernize his empire.

If he breaks the law then he had to resign from his post and so he'll lose his/her powers.