Duh! Why the hell didn't he go back, say, two weeks before and warn his brother about what will happen? (Shitty script-writing, that's why.)
Or excellent script writing, true to character. Picard knows about meddling with time travel, he's not a god, he can't go around fixing every problem he's ever encountered. Going back to the moment that caused all of this changed nothing, no damage was done, that's the most logical thing to do. Very in character with Picard.
(Also, hindsight being what it is, we've had how many years to think this over, where Picard was in a cloudy Nexus and had only remembered what the mission was)
Sorry, I don't buy that for a second. First of all, if you want to talk about Picard's character, I think almost every TNG fan out there would agree that Picard of the movies isn't the same guy as Picard from the TV show. But I'm not even going to go there.
Not only did Picard lose 2/3 of the remaining Picards in existence, not only did they die in the most horrible way imaginable for someone to die...but a Nexus-induced fantasy of Rene and what could have been a new entire line of Picard kiddies is thrust in his face, right before he finds out that he can go anywhere at any time (and without the benefit of being a god, to boot). So, uh, wouldn't he want to go back to where he "really" could make a difference, for everyone? Like, say, saving his brother and nephew from a completely meaningless death, stopping Soran before he ever even started causing stars to be destroyed, saving his ship from meaningless destruction, or, heaven forbid, going back even farther and having that family that the Nexus showed him?
This has nothing to do with Picard's character. This has to do with being human and making the most logical decision when one is given the opportunity (in the face of ongoing danger) to change history for the better.
And it is sloppy writing, without a doubt.