Okay so you have three wads in two zips.
pet containing pet2.wad (E1M1) and pet.wad (E1M2). Yes, they're out of order. You're supposed to load both at once - I guess they didn't have wad combiners that early on.
- pet2/E1M1 is just a square box with a lift and a bunch of troopers. You're supposed to get the chaingun to do the real map. Getting 100% kills here is interesting as there's a couple of sergeants in alcoves at the top of a really high wall, and you must shoot the far one from a moving lift.
- pet/E1M2 is your typical oldschool map, with a completely random theme, illogical lighting, fullbright to complete darkness transitions, higgeldy-piggeldy linedef placement, all that good stuff. More unusually for an early level, it is quite hard.
petkillr, which replaces E1M3, is a harder version of E1M2, with an extra maze and about twice as many monsters. You can play spot the difference, yay! It just gifts you the chaingun for free without any messing about with pointless introduction maps.
What intrigues me most about this wad is the ending - it looks for all the world like an E1M8 thing where you have to kill some barons to lower a wall. This is especially true of pet.wad which doesn't have any other barons in the rest of the map. On the other hand in petkillr it'd be really annoying to get to the end having missed some and having no idea why the exit isn't opening. You aren't forced to get 100% kills, indeed you can get to the end without seeing the baron that guards the plasma gun.
There is also a cyberdemon that is trivial to dodge. So then you have the version of petkillr in wadpak2. You have to leave the thing alive and come back for it to get 100% kills here, because otherwise the map will end halfway through - it replaces E2M8. This seems to be a deliberate choice by wadpak2's compiler, I doubt he'd put it at E2M8 by pure chance. Very strange.