Buddha. The Savior Servant.
Being a Buddha means that one has attained enlightenment.
The Master who summons this servant, regardless if they are good or evil, must have awakened a strong desire to “save mankind.”
The most famous Buddha, Shaka, achieved enlightenment on his own. While still alive he was liberated from so-called “suffering.” His “teachings to attain Buddhahood” are what makes up Buddhism.
However, in Buddhism there is nothing that says, “If you study this you will be saved.”
Buddhism is the collection of principles that Shaka followed to reach enlightenment, and Shaka never proclaimed that his teachings were the only path to attain Buddhahood. The path to enlightenment varies from person to person.
As far as Buddhism is concerned, the potential to become a Buddha is present within everyone. Every human will eventually attain enlightenment. Of course this includes those who are good as well as those who are evil, those who are kind and those who are heartless; everyone is subject to the process of becoming a Buddha.
The reason that Saver answers the summons to become a servant is that its Master’s suffering is too great and cannot be ignored, and also burdened with sins that are too great.
“There is no way for this soul to be saved.
Well then at least until he has realized the result he desires for the human race,
I will walk alongside him.”
Saver has no desire to lend his powers to the Master, he only cooperates out of mercy and benevolence he feels for the King of Conflict.
…For this person who has been constantly embroiled in battle from birth and till death. Savior will become a shield for the final chapter of this poor soul who despite resenting war, was unable to deny it, and forced to walk a pain-filled path. Like the flowers of a saul tree.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA