The DLC is so much worse than the main game, it almost ruined my experience

Battle Out of Hell was never a DLC. It was an old-school retail expansion pack that sold on the shelves of brick & mortar stores.

I don't agree that it's worse than the main game, except for in one regard: they did a very half-baked job on the boss levels of the expansion. However, the main game is where that trend started, because its final boss/level (the battle vs. Lucifer) was equally half-baked, and by far the worst level in the main game.

Other than those weak boss levels, Battle Out of Hell is excellent.

Can you be more specific about your criticisms of Battle Out of Hell? What, exactly, didn't you like about it?

By the way, are you sure you are talking about Battle Out of Hell? Because Painkiller has a lot of bad retail products associated with its brand (many of them were made by modders, not professional developers). Possibly you are mixing up Battle Out Of Hell with one of those bad modder-made games?

What I didn't like is the fact the checkpoints are way too far apart and thus forcing the player to savescum. The two rollecoaster rides are just plain awful. There is way less enemy variety and the levels are much less interesting to traverse. A lot of people criticize Painkiller for being just a series of death rooms, I wouldn't agree when referring to the main game but BOoH is definitely that. Leningrad is just stupid, what's the point of turning the game into a cover shooter if you don have any cover systems? Every level in BOoH has at least one textbook mistake on how to make the game worse, it really feels like all the bad ideas that got cut from the main game mashed together

I didn't notice the levels having less enemy variety. I.e. the clowns only appear in Loony Park, the zombies only appear in Dead City, the undead soldiers only appear in Leningrad, etc. That seems to indicate a lot of variety IMO. Contrast that with a piece of crap game like Doom 3, for example, which features the same few monsters (i.e. imps) repeated endlessly, ad nauseum, on every level, for the entire game, from start to finish. And Doom 3 also has almost no variety in its environments either, even though PK and BooH have tons.

I love Loony Park and Leningrad and Dead City. I found all of those environments to be fascinating. Yes, they are all challenging levels, but IMO that's a good thing. Although I can see how not everyone would like their high difficulty. Maybe playing them on lower difficulty setting would make them more fun for you? And/or using better Black Tarot cards that are best suited to each individual level?

As for needing to "savescum," that's what the quicksave and quickload keys are for. They make saving & loading fast & easy.

I agree that the roller coaster is stupid given that it leaves the player with no room to dodge. On the other hand, it also motivates the player to make good strategic decisions in regards to a) which Black Tarot cards to take to beat the roller coaster segment and b) not to use the cards earlier in the level, before that segment comes up.

The point I took from Leningrad is that war is Hell, both figuratively and literally. The high difficulty of that level is the main thing that contributed to the feeling of terror I had whenever the music started to play (which was also a unique touch). I really did feel like I was in a war and that I had to fight for my life during every second and for every inch of territory over which I advanced. Definitely the tank vehicles (which is yet another unique touch) and some of the enemies' sniping weapons are way OP though.