"Decider" is not the word I am looking for. It is used only in context of a game. Can anyone help me with a word for "one who decides (something)", in general context?
Well, one phrase that is commonly used is "decision maker." There are other, specialized words or phrases for many domains. By the way, the word "decider" is not only used in the context of a game - for instance, George W. Bush said of himself (when explaining his view that every process has to have one final decision maker) "Well, I'm the decider."