Rude and terrible behaviour! As a guy in a busy gym I often have people asking for how many sets left or if they can jump in but in general people are quite respectful. Recently I had a guy about to throw a tantrum when I said I just started on the bench but he caught himself and left it at that.

However your story about that guy who just jumped in as in you weren't there... that's on another level. There's a special place in hell for these guys, along with people who don't put their weights back.

This reminds me as well of the one time I was using the squat rack. A woman came by and asked if she could jump in for a bit which got me quite excited as there was quite a bit of weight on the barbell at this point and I thought she was going to squat that weight.
... Instead she put her foot on the barbell and started doing her stretches... could have done that anywhere else and the gym wasn't even busy. Took her time too as she started chatting with some of the guys around the squat rack.