The Borg and Q were responsible for Jennifer's death. Picard had nothing to do with it. Picard didn't go out "looking for the Borg". Q showed up and was being a dick as usual, and sent them there to teach them one of his lessons.
I actually registered here just to correct you on this. You seem to have missed the entire point of "Q Who?"
Picard was arrogant and told Q that he didn't need his help as they were ready to meet whatever they would encounter in space. Q flung them thousands of light-years away to put that assertion to the test. Guinan warned them to head home immediately as this region of space was dangerous. Picard ignored her warning and started exploring the area. The Borg cube showed up. Guinan warned Picard that they are very dangerous and cannot be reasoned with. Picard disregarded her and tried to communicate with the Borg. The Borg kicked the Enterprise's ass and Picard realised he was being arrogant and begged Q to send them home. At the end of the episode Picard realised that the Borg would be coming for them now and it was all because he was too arrogant.
These events led to the Borg attack at Wolf 359 which killed 11,000 people, including Jennifer. So even if we ignore the fact that Picard was one of the Borg on the ship which killed all those people, he was still responsible for the ship being there in the first place.
Sisko had the right to be pissed at him.