Rutherford had a military career, and many such people keep a firearm as a souvenir (or for sentimental reasons) after they are discharged.
The three latter victims could have run away to other levels in fear after the first gunshots, maybe they tried to hide realising they couldn't reach the door to get outside?
It sounds like his 'fidgetty' nature and other nervous habits made him the type of guy who would let things build up (fester if you like) until it got the better of him.
Yes, it was premeditated in the sense that he was going to do SOMETHING dramatic, then a few gins later, it all reached a crescendo.
Good point Dave, He may well have been fixated with the Simcox woman, but never told a soul.... until that day, then all witnesses and the scene had to be eradicated.