Mark St. Pierre, the driver who injured 13 people and fatally ran over a father walking with his son in the Bronx last month, has been formally indicted on a charge of second-degree murder, and faces up to 25 years in prison if convicted. He appeared in court yesterday to plead not guilty to 50 counts related to the appalling road rage rampage, which resulted in 6-year-old Sebastian Colon seeing his father Miguel die before his eyes. The two had been crossing East Gun Hill Road in Baychester when St. Pierre sped toward them, leaving Colon just enough time to shove his son out of the way. A witness tells the Post that St. Pierre, who had just had a fight with his ex-girlfriend at Chuck E. Cheese, was doing "at least" 85 mph when he slammed into Colon. After the argument, he sped off in his 2008 Infiniti, crashed into two parked cars before killing Colon, and then proceeded to hit several more cars until his vehicle was too wrecked to continue. At the arraignment yesterday, Colon's widow told the Daily News, "My heart dropped when I saw him. To me, I am staring at an animal."