A regime with negative actions is often supported by a leader who is enacting negative influences and oppressing the population. By assassinating that leader, the leader is removed from power and those negative actions cease to affect the population. It also sends a message to other potential tyrants that death could meet them if they are acting as a tyrant.
When it comes to what one may think would lead to a "positive change", murder/assassination is never the answer. There are for better ways to solve a problem then taking ones life away from them, just because you do not agree with their particular opinions of certain subjects. Murder does not solve anything.