Shinji becomes a Rider because he won't do what his boss asks him to do. It'd be one thing if he wanted to investigate the disappearances for some greater purpose, but I've never gotten that impression. As written, the first episode has always seemed to depict Shinji as merely curious to me.
You can argue he tries to use his power wisely from there, though I think Ryuki as written intended viewers to look at Shinji more cynically. I think Ryuki is the kind of show that condemns a character for trying to save others when he can't really save himself.
At the end of the day, I think it's debatable as to whether Shinji saved people because it was the right thing to do, or because he'd rather play the hero than play the villain. Motivation in that case opens up a great big can of Kantian ethical worms.