Murder Madness by Murray Leinster (1896 - 1975)

Murder Madness! Seven Secret Service men had completely disappeared. Another had been found a screaming, homicidal maniac, whose fingers writhed like snakes. So Bell, of the secret "Trade," plunges into South America after The Master--the mighty, unknown octopus of power whose diabolical poison threatens a continent! (Summary by Murray Leinster)