Clearly, the dictator is a monster, however his supporters will remain loyal to his cause, he will have created a propaganda image for himself which resonates with people. The opposition to the dictators would have succeeded in destroying the 'monster' at the top, but still need to tackle the foundation laid beneath him.
Assassinating a dictator is a good means to affecting change. When a dictator dies in such a public and shocking fashion, people have feelings stirred up within them that will inevitably cause change in their countries. While the aftermath of the dictator's death may seem a little chaotic at first, people will soon begin to enact change.
Most dictators are in power because they have a cadre of supporters that keep them in power. Removing a dictator by assassination would just lead a power vacuum that would just be filled by another person. You can't just "cut off the head and the body will die" when it's a monster capable of growing another head.