1 Dolores became conscious
Dolores solved the maze, became conscious, and was able to make her own decisions. It's widely believed that Dolores killed Ford intentionally.
2 Dolores realized she is Wyatt
As Dolores became conscious, she realized she is also Wyatt. Dolores realized she can kill just like she used to do when she acted as Wyatt. She even used Wyatt's gun to do so. See Westworld plot explained - Season 1.
3 To avenge the hosts
As Dolores realized that the hosts were suffering because of humans, and specifically Ford, she took the conscious decision to kill him in order to free the other hosts from his control.
4 Some believe she was programmed
Some people believe that Dolores was programmed to kill Ford and the other guests by Ford himself.
5 Ford might have been a host
According to another fan theory, the Ford who was shot in the last episode of the season might be just a host resembling the real Ford who has other plans. See also why Dolores killed Arnold.
6 To follow a bigger plan
Many fans think that the killing of Ford was the beginning of a big plan set by Ford himself in order to help the hosts escape from the park. See also what happened to Elsie.
7 Ford was aware he is going to die
For was pretty aware he is going to die by the hands of Dolores. He either predicted she will make that choice or programmed her to do so, according to the other theory.