IIRC (and I might be garbling the memories) Jason, when last seen, had been about to cut the Joker's throat and was daring Batman to kill him to save the Joker's life. Boom... bye bye (of the ominous sort)
Next time we saw him was in COuntdown when he was trying to stop Joker's Daughter from making a kidnapping, saying he was trying to reform because... well, if there was a reason given I have no idea what it was?
Next thing we know Joker's Daughter is killed (by a Monitor) and Jason for no apparent reaosn decides to make this HIS case, to the extent of interacting with the Titans and Teen Titan about it, and Donna is "We should trust him because he's said he's trying not to kill people anymore (and we'll just forget all those people he HAS killed so far)".
At this point my tolerance snapped and I decided Countdown could do without me.