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In the DC comic book series, Birkin is seen slaughtering animals at the Raccoon Zoo before killing two employees in a violent rampage.
Oddly enough, Sherry mentions to Claire that her dad calls out for her, asking for help as he is attacked by "those monsters". However, in game, only his groans can be heard; he would call out for Sherry in Resident Evil 1.5 and the comics, though. However, in the Darkside Chronicles, he calls out Sherry in the middle part of the game.
Also according to the leaked audio, he seemed to be nervous and aware that someone was after his research. Although one of the researchers knew what it was, he didn't touch the subject.
Birkin is the one who gave Wesker the experimental virus in order for him to fake his death in the Mansion Incident, causing him to be reborn and gain superhuman abilities. It is unknown if Birkin ever heard from Wesker after his resurrection.
Ada Wong obtained info that a researcher was close to the completion of the G-virus but refused to hand it over to Umbrella. Wesker knew that the researcher was William, and planned to save him and present the G-virus as a gift to The Organization. Their plan was foiled by the U.S.S. who stole the virus for Umbrella and killed Birkin.
William Birkin was originally to have a much bigger role in Resident Evil Outbreak in his G-1 stage, appearing in an Umbrella facility. This was shown in the Resident Evil Online trailer. However, when the production had to be scaled down, he was ultimately removed in all but the opening cutscene, though his in-game model was kept.
When Brian Irons confronts Claire in his taxidermy room and divulges information about the G-Virus while threatening her, he implies that Birkin had a hand in the development of the T-00 (referred to as the monster tearing the precinct apart) due to it being a product of the G-Virus. This was actually an error in the localization. In the original game, Irons simply explained that it rapidly induced human evolution, without even mentioning the T-00 at all.
William injects the G-Virus into his body differently in both games. In the original Resident Evil 2, He injects himself with the G-Virus on his stomach as his eyes became red at the start of his mutation but on the Darkside Chronicles, He injects himself on his wounded left arm as he grins evilly before being mutated to his first form.
In the original Resident Evil 2, Birkin dropped a mug, which startled the U.S.S to shoot him.
William's death in the original Resident Evil 2 had him defeated by whoever was used in the B Scenario. As Birkin's viral presence caused the train to auto self-destruct, Leon, Claire and Sherry fled the train after successfully using the emergency brake to stop the vehicle. After the trio leave the tunnel, the train detonates itself, killing William.
When William injects the G-Virus into his own body, it was different in the three games.
in the original Resident Evil 2, he injects the virus on his stomach as his eyes glow red.
In The Darkside Chronicles, he injects the virus on his incapacitated left arm and evilly grins before mutating into his first form.
In the remake version, he injects the virus on his chest out of anger and frustration for losing the G-Virus while he slowly mutates and walks off, witnessed by his wife Annette.