Everyone's supposed to get to hardmode because it's just the halfway point of the progression grid, it's not literally an optional hard mode that you can turn on or not if you
wanted to
There is no save file ruining with these enemies, if anything you just need to wait until you get more bonkers weapons and minions so that you more outclass the base level enemies in hardmode since now you only have base level hardmode equipment
1) What difficulty are you playing? (Classic/Expert/Master. First/blindish runs probably deserve Classic)
2) How many hearts do you have? You're supposed to mine the heart crystals and use the hearts to get up to 20 hearts for 400 MaxHP, with this much you shouldn't be getting one-shotted by anything even if you had NEGATIVE points of armor, I'm assuming you accidentally killed Wall of Flesh early where you have very few hearts
Technically it's
possible to skew enemy spawn rates if you make 3 simple rooms together and place 3 NPCs there, so now monsters won't appear when you're here, can even fish in peace
The thing that sucks about it and bothers me a lot is that beating Wall of Flesh for the first time puts the corruption and the hallow a bit to the sides from the middle and now those 2 will just keep spreading over everywhere. Literally the only way to stop this is to go over the surface until after where it stops, then dig straight down to underworld so it won't get past the hole. (a 3-wide hole works, but blocks that grow grass up need to be stamped/walled over to prevent floor vines passing over, or the hole just needs to be 6 wide. Or you could just buy stacks of dynamite, gel them for sticky dynamites, and get it over with)
Later you can buy a spray thing that undoes any corruption thing and it reaches about the entire screen, but the area needs to be dug 3 blocks isolated (or separated with something like 3 wood) from elsewhere for it to stay pure otherwise it will just grow back again from touching the rest
(also for some reason, using the pwnhammer to destroy altars can put corruption on a random block in the map, literally out of nothing. Fishing crates work as an alternative to getting their ores instead)
If you're playing on journey mode character&world, there's a power setting to disable the spread altogether. It also has other settings like lowering/disabling enemy spawns or even unlimited invincibility so maybe you might want to play with that
I breathe in ketchup and breathe out anxiety