Petyr is unpredictable and seems like a vampirish type of senile (e.g. referenced when the others use his age as a reason to not include him in housemate meetings). He's capable of restraint, on a good day, but has "senior moments", and we see a few of these like his occasionally lashing out when the others go to visit him. If I remember right, he looks bashful after turning Nick, which suggests it was some kind of lapse, not something he had thought through.
Petyr also has history of spontaneously turning complete strangers, unlike the more modern youngsters who practice discretion and family planning:
Deacon: One day I was selling my wears, and I walked passed this old creepy castle. And I look at it and think, "'very old and creepy". And then this creature... flies at me! It dragged me back to this dark dungeon. And bit into my neck. And just at the point of death; this creature forced me to suck its foul blood. And then it opened it's wings, like this. And hovered above me. Screeching. 'Now you are vampire.' And it was Petyr. And we're still friends today.
So it had to be some kind of deliberate act since it involves making the victim drink the vampire's blood before they die (unlike the
swearwolves werewolves turning Stu) - but, spontaneously turning complete strangers appears to be an old habit for Petyr. He's lapsing into old habits from his youth: a vampirish version of a 'dirty grandpa' inappropriately flirting at a waitress.
The other vampires don't take Petyr out hunting, don't bring humans home for him, and we see them feed him by throwing live chickens into his lair: they seem worried around him that he'll do something inappropriate.