Ignore the fans that are trashing Pearson on his way out. He's not nearly as bad as you may be led to believe. He's simply not producing this season... on a team on which few players are and the mood is heavy. That doesn't excuse his performance, but it suggests that he could bounce back. Getting out of that environment and a chance to start over is a blessing for him. I'm not sure if he'll hit 24 goals again (that was due to a perfect storm of line chemistry and confidence), but I'm sure that he can be a 15 goal scorer again, if not this season, then next season. If he can't stick at a 2nd liner, he should always have a place in the league, at least as a 3rd liner, since he's at least decent at just about everything. He's a very respectable player if you don't expect too much from him.
I don't think it is fair to trash or praise Pearson. The objective facts are that he was a promising 15-20 goal scorer early on in his career, with promise of being a strong 2nd or 3rd line forward. His play has significantly dropped off the past 2 years. If his confidence goes up, he is in a different environment or gels with his new linemates, he could be pretty good again. It's a bit of a risk for Pittsburgh, but not a tremendous one.
From LA's perspective, if they held onto Pearson longer and he continued to not score then the trade prospects could completely eliminate. They would have to keep Pearson (as a negative value asset) with the hope that he would rebound next season. If he turned his game around, however, they could get a better deal for him. Blake decided to take the safe approach and get him off the books.
How good Hagelin is or not is not important for the Kings. He won't be hear past the trade deadline. Even the Kings' mouthpiece, John Rosen, wrote an article today stating that he is likely gone by the deadline, which says a lot. Rosen doesn't usually say anything controversial because he is just the Kings PR guy.
This trade is a risk for both parties. The winner remains to be seen.