I call shenanigans (spoilers)

Yeah the killer is almost superhuman...with cops only 2 mins away, he not only casually taunts Malloy on the phone but also kills Mary brutally, hangs her body, escapes while at the same time setting up a trap for Malloy to chase him and trick him into shooting the cop (who he captured when!!?)

About the scene you mentioned, there is a split second car scene in between when Slater is in the room with the doctor chick and the crazy mexican and when he electrocutes the old guy, followed by another scene where Malloy is chasing the screams from his electrocution and lights dimming, before he comes to the room where the group is now congregated, with Slater present there too!!!

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wat are you lookin' at...