And as an aside, I think they got the ballistics right -- at that range, and having gone through the window pane, the bullet probably wouldn't have the energy to exit the skull. At a more normal range it would've gone right through.)
now let's not get carried away - the explanation that the bullet flies at the speed of plot makes sense, but any mention of ballistics stretches it about 200 meters too far
when the game does attempt something resembling "ballistics" or firearm mechanics, the caliber mentioned is "4.46" (cute). now i don't know what kind of unicorn superglue they used to make it mechanically sound in the p&p ruleset, but that caliber strongly implies a low-weight high-velocity round that penetrates flesh like the needle it is but isn't famous for stopping power, and loses ~80% of its kinetic energy (and drops more than 1m from sight zero) at 500 meters
imagining that kind of very thin but long paraboloid of lead striking a window pane after most of its energy had been depleted and not tumbling immediately - but piercing the glass and throat-shotting the merc is... beyond my (arguably limited) boundaries of verisimilitude
if it had it been an (imaginary) "8.62×53mmR" caliber round it would work though