"Witness the victory of the echo over the voice." - Shinzy had a HUGE inferiority complex, because of his shitty life in Reman mines, because he's "just" a clone. And through his incredibly twisted worldview, saw the solution in utterly annihilating Picard's life, his world, any trace of his comparitively cushy, easy, privileged life. He wanted to bring humanity down to his level, so to speak. Show them what it's like to live without comforts of Earth and the Federation.
Is it really any less believable than Hitler's quest to exterminate the Jewish?