Warning: loads of death. Reader discretion is advised. Hoo-hoo-hoo-hoo!
So, instead of a oneshot, I'm going to make scenarios that Barnaby would probably kill people.
Me: Because of my autism, he would probably catch me off guard in a very public setting. He might try to mimic my mother or brother, making them demand unrealistic expectations. Or try to make me drown while sailing (and fail miserably because I know how to swim away). Or get my anxiety to deadly levels somehow. Or cause my laptop to explode. Puns he would use: "Shocking, shocking! That is electrifying!" (laptop), "You have enough panic to power New York!" (anxiety attack), "(sings a sea shanty) Oh? I have a sinking feeling!" (drowning).
Harry Potter: A simple "Avada Kedavra" would be his method. And he would probably use it on so many people. Puns he would use: "Hoo-hoo-hoo! So you wanna play with magic?" (death by killing curse)
Poppy Playtime: Barnaby's first method would be flooding the abandoned factory with laughing gas. Or... in Catnap's case, set him on fire. Better yet, he would probably cause an explosion with the red smoke. Puns he would use: "(sings Light Em Up by Fall Out Boy)" or "I love the explosive energy!"
Bones: Barnaby would definitely screw around with chemicals due to the fact that the setting is an FBI lab in the Smithsonian. "Here's a chemical peel!" (death by chemical)
NCIS: Does not matter if it is in New Orleans, Washington, L.A, or Hawaii. Depending on the case, either gunshot, drowning, or some outside factor.
Five Nights at Freddy's: Well... for the missing children or the people haunting animatronics, Barnaby would not have to do anything aside from offer relief after revenge by just bringing them to his party as is. As for human characters... he would probably use the animatronics after they are abandoned, or other factors, like monitors exploding. But he would leave a small heads up that time is out: he would possess the computer, put a message on the screen like "You are invited to an eternal party" and cause it to combust. But Springtrap specifically, all I can say is: a combination of acid, fire, laughing gas and springlock failure.
Portal: Barnaby is definitely going to have a field day here. There are high energy pellets that he can manipulate, pushing unsuspecting test subjects into lasers, or get in front of turrets to have them all shoot. And there is also shoving test subjects into the water while carrying portal guns. If someone avoided him too much, he would use the deadly neurotoxin. Or cause explosions. Puns he would use: "Sorry, I thought you were Elaine!" (death by laser), "Sink or swim!" (falling in water).
Half Life: He would use all of the monsters to his advantage.