Once Karl and Theo arrived in the front of Nakatomi Plaza, they got out and entered the lobby, when Theo was talking about basketball to keep the front guard from getting suspicious of them. Karl casually pulls his suppressed Walther PPK pistol and kills him with a gunshot to the head. While Theo was sealing off the building, Karl uses a hockey puck-like flashbang grenade on the other security guard, momentarily blinding him. Karl takes advantage of this distraction and he jumps out from behind a corner and shoots the guard several times in the chest, killing him.After Karl helps seize control of the building and the executives in the 30th floor celebration hostage, Karl held the pistol out for protection as his leader, Hans Gruber seeks out Nakatomi top executive Joe Takagi. Once they took Takagi up to the 34th floor, Karl, along with his brother Tony and Theo watched on as Hans kills Takagi after refusing to give him the code to the vault. When they heard a strange noise, Karl lets out his Walther PPK as he and Tony searched the room to investigate. When they couldn't find anything, Karl holsters his sidearm, not knowing John McClane, who caused the noise while fleeing, has evaded them. The weapon wasn't seen afterwards, although Karl did grab for it when Harry Ellis obnoxiously entered Holly's office, only for Hans to tell him to stand down.
Another Walther pistol, a Walther P5 is seen briefly in the film as well. Walther was famous for making many variants of the PP in various calibers, including the PPK, PP/S, TPH and the P5. The P5 is essentially the same gun as the Walther PPK, except for it is the full-sized variant of the pistol and is chambered for the much larger 9mm cartridge, as the .380 ACP cartridge is just the scaled down version of the 9mm cartridge. The Walther P5 is Heinrich's main sidearm, and is seen when Heinrich tries to kill John after he pulls his Beretta 92F on Marco. He shouts for Marco to get down and aims the P5 at John, but John is faster, and blows Heinrich away with his pistol before Heinrich can get a clear shot.
The gun's appearance in the film would make sense, as it is a German firearm, as Heinrich was a German terrorist and Walther is a German gun company.Katya's main weapon is a custom nickel-plated Walther PPK/S with pearl grips. The PPK/S is essentially the same gun as the Walther PPK (similar to John's Beretta 92F and FS). The only difference is that the gun is made of a Walther PPK slide on a Walther PP frame, to create a bigger firearm than the Walther PPK for American markets. After Mathias Targo reveals to Katya that Simon wasn't planning on sinking the gold like he claimed, instead planning on keeping it for himself, Katya draws the pistol, apparently to kill Simon with it, only to surprisingly to turn the weapon towards her friend, Targo, and emptied the weapon into his chest as a show of her loyalty to Simon, killing him.