We can probably thank human biology for that, as it's not as simple as pointing and shooting.
Engaging any large living target to kill causes a massive adrenaline rush, even with training. I don't know if you've ever been a deer hunter, but "buck fever" is real and very difficult to suppress even for the most seasoned hunter.
I hate to even give the impression that I'm drawing a comparison between game hunting and mass shooters, but the point is I imagine that the flood of adrenaline in your system that makes you jittery and makes your hands shaky as fuck sometimes when you're hunting is amplified tenfold in a situation like this, unless you have had A) a complete psychotic break in which your brain is not functioning/producing hormones normally, and/or B) extensive military training.
Someone like Charles Whitman would be a good example of both.