Fringe/Recap/S03 E11

Season 3, Episode 11:

Reciprocity
Olivia, Peter and Walter are brought to the Massive Dynamic laboratory where the Wave-Sink Device has been assembled. No one has been able to access any of its functions, but when Peter gets close to it, the machine actives of it's own accord and Peter suffers from a nosebleed.

Soon after, Fringe Division is called in to investigate a series of murders after the victims are discovered to be Shapeshifters. Broyles and Olivia believe that it's the work of another Shapeshifter, cleaning up the network to stop Fringe Division from capturing and interrogating any of their number, and that cracking the encryption on Fauxlivia's laptop might help them track down the rest of the Shapeshifters in the prime universe. However, it's Walter who is first to discover the truth: the killer is Peter, who already cracked Fauxlivia's laptop and is recovering their data discs. After figuring it out, Walter arrives too late to stop Peter from killing the last Shapeshifter, but doesn't give him up to Olivia and Broyles.

Later, Peter claims that he is attempting to be proactive in the fight against the alternate universe, and that Shapeshifters aren't real people, but Walter theorises that the machine has somehow "weaponized" Peter.