Why Dolores killed Arnold on Westworld?

Arnold wanted to commit suicide and to close the park after realizing that the hosts have feelings and that they can truly suffer. He controlled Dolores and then made her kill him.

Ford made it clear later in the show that Dolores wasn’t the one taking the action. He said that it was Arnold doing it using Dolores’ body.

Before his death Arnold programmed another personality into Dolores , Wyatt, which has the ability to kill humans ruthlessly. Arnold probably used Wyatt’s side of Dolores to kill himself.

After failing to convince Ford that the hosts have feelings and consciousness Arnold decided to commit suicide. He believed his death might prevent the park from being opened. (See Why do people binge-watch TV shows?)

If Ford was not telling the truth then something else could have happened to Arnold. Since Ford is the only one who knows the real truth it’s not possible to know whether that happened or not in season 1.

According to some Fan theories Ford could have been the one who killed Arnold , using Dolores, because he had different intentions for the park. (See Why 13 Reasons Why TV is a good show?)

According to another fan theory Dolores might have been conscious while doing that action. In such a case Dolores could have had her own motives.