1 I will assume you play as flash early on. In his story he fights Batman's allies and comes to not like the fact he is fighting fellow heroes, causing him to switch sides. He finds that
2. Green lantern, or yellow lantern atm, is fighting batman. He doesn't arrive in time to stop the fight where Hal, apparently, kills batman. Distressed over this the two fight, with GL the victor. Having just fought one of hiss best friends, Hal finds the error in his ways, becoming the Green Lantern once more to confront Superman.
3 NIghtwing, seemingly distressed over his mentors death, joins Deathstroke (NWs alt looks more like DS's armor to me), for a chance to get Hal. To prove his loyalty, Slade has him either a) Retrieve Batman,s Kryptonite from the Batcave, or b) beat the hell out of Green arrow, as the two of them are not on good terms. Either way leads to NW fighting GA.
4) Eventually, the villains turn on the heroes, which Batman had expected the entire time, Having his Death, and Nightwing's Allegiance as ploys, with this enough of a surprise that the Heroes gain an upperhand on the villains.