I'm not saying that a law wouldn't stop them. Science would be much more effective. Take jails and prisons for example. An individual with genetic markings for crime or a mental illness is not going to be "rehabilitated," so spending thousands of dollars to incarcerate criminals will never truly rehabilitate some of them. Jail will rehab some people, true. Others - drug users, murderers, psychopaths - will someday be prevented through genetic identification of certain markers indicative of illegal activity, making jails and prisons a thing of the past.
To go back to my original point, though, laws aren't necessarily useless here. The Colorado shooter purchased most of his weapons and ammunition legally. Who knows if he had the means or knowledge to purchase these weapons illegally? A law might not have prevented it, but if doctors had identified certain genetic characteristics before he was even born, he could have been prevented buying through legal channels, possibly tried to buy an illegal gun, and gotten arrested. At the very least, he'd be on some kind of watch list for potential crime.