It's hard to say what Lyn would do if they turned out to be the murderer. If she went after you, Umbreon, that would be obvious. It would depend on how sly she is and whether or not she would risk trying to kill off the person who accused her.
I've been editing that post a lot of times, so I'm not sure if I got that last paragraph in while you were reading.
Then again, if she IS the murderer, than she's in a lose-lose situation. She kills me, she gives herself away. She doesn't, I would uncover more and more weird things against her when she starts appearing more in murder scenes.
If she isn't the murderer, I'm quite stumped unless it's Bagu. He's the only person who was at the scene other than Lyn and Pidgeotto.
That said though, you brought up some really good points. It didn't specify what Bagu's reaction was; he could have very well have been shocked. That said it wouldn't be the first time someone screamed after seeing the dead body "for the first time" so it doesn't look like their the one who did it.
I kind of want to lean towards Lyn here myself because she's the less obvious choice. More sneaky you could say. But at the same time the room went pitch black. If it was electricity that killed Pidgeotto it would have brightened up the room. Either way these two may not even be the murderer as the culprit could have easily killed Pidgeotto without having to be right next to him. Hnn...
Damn, I forgot that electricity brights up stuff. Wait, but if the others were pretty far away, they wouldn't notice the shock, would they? And all of them were probably asking out loud what had happened, so they couldn't hear the static. But those are pretty bad points, it's likely that they'd notice anyway.
But then again, WHY did the lights go out? That's a very important question. Power cuts don't happen often if you know how to use your electricity, and it also can't have gone out on it's own because the whole mansion went out. There's only one conclusion: Lyn sabotaged them. While they were out, Lyn couldn't have used Electric attacks as they would have brightened up the place, as you said. However, the possibilities don't stop there.
Lyn could use Ice Fang. Very effective against Pidgeotto's type. And not only that, if Lyn DIDN'T have the move, she must've had Bite instead. Not only does it not create any light, but it could've easily caused Pidgeotto to flinch all the way through the murder. Even if Pidgeotto didn't flinch at one part of the murder, Pidgeotto wouldn't know where to escape, so it's a goner anyway.
This is how the events most likely happened...
The Pokémon decided to explore the mansion in groups, well, at least Bagu and Lyn anyway. What the others didn't know, was that Lyn had a fiendish plot to kill Pidgeotto, as they probably hated each other through and through... And eventually, Lyn was in luck, as she and Bagu managed to find Pidgeotto. Pidgeotto greeted the two firmly, and turned back to meet the others when Lyn sabotaged the lights and started biting Pidgeotto repeatedly, while Pidgeotto flinched for most of the time and tried, but failed, to run away. Lyn was pulling him back with more of bites, as Bagu looked at the scene, frozen in horror. Then, as soon as Pidgeotto was dead, Lyn quickly made a note, placed it on Pidgeotto and gave a giant scream which caused everyone to arrive at the scene, which is where we are now. This is the reason Bagu's expression didn't change. He had witnessed Lyn kill Pidgeotto slowly. Then, Lyn used an Electric attack to put the lights back on.