THUMBS DOWN to the Dallas grand jury that cleared TODD ALAN BROOM, who reacted to a cold-blooded murder at Irving Mall by committing one of his own. It’s tempting to say that Broom’s victim got what he deserved, saving the state the time and expense of a trial. But would Broom-would anyone-want a society in which each citizen meted out justice on his own? In our heavily armed society, letting Broom off without a trial sends an ominous message. Honk if you want someone making deadly snap decisions about you.