book about a man who built a device that could influence people

It's not about people going insane, it's about an invisible ray that transmutes gold and kills people, but this is very similar to "The Mystery Metal" (1930) by H. James and Maurice James. It was published in Science Wonder Stories, March 1930.

"Here," he pointed out, on a map of the city, "is the location of the Interstate Bank and Trust Co. Draw a line through the vaults and the place where Tom was found dead; it runs like this." He made a mark with a ruler. "Then here is the location of the Hollingsworth National. Draw a line straight north — the three men fell dead on the north side, in line with the vault, you know. The two lines intersect at 332 Grosvenor Place."

"It is from there that the fatal rays have been directed! Furthermore," he explained "the fiend must have his instruments on the eighth floor; otherwise it would have affected other persons in the path of the rays. Directed from higher up, they would not have struck the men in front of the Hollingsworth Bank. Directed lower, they would have affected others."

In this story we don't find out the motivation of the scientist, John Kay, who is using the ray, since he is killed by misadventure before the heroes arrive. You can read the entire story at the Internet Archive.