Which New Game Plus are you talking about? The first one?
Yeah, it takes some getting accustomed to. If you just zipped through the game the first time around, it will feel - and quite probably be - much harder. If you took your time to get good at handling (and upgrading) your favourite weapon and gear, memorizing distances and maybe even perfect the art of parry-riposte, it will be much, much easier.
The darkness is not that impenetrable. There are at least three proper solutions to that one problem, and then there's always fatalism. Just map the area by running in one direction. Resort to age-old maze/labyrinth/dungeon basics. Hug the wall. That's the thing that blocks you rather than inviting you to fall to your death. The first time around, everything is lethal and brutal and not quite fair. Once you've seen it all, it's all about memorizing... well, everything. That's why ploughing through ranks of enemies, dropping them all without getting so much as sneezed upon so very, very rewarding.
Besides, since dying is basically free and your status as the guy with the halo Darksign gives you infinite lives for as long as you don't hurl yourself out of the window, you're really just invited to figure things out. If one approach does not work, try another. If nothing works, start right back at the beginning again. Maybe there was something you missed. I think it's absolutely alright to start out dying a lot. As you progress through the game, though, perfection is pretty much your bestest friend.
in hundreds of hours of playthroughs, walkthroughs, strollthroughs and free walkabouts, I've only killed one NPC so far. Because, when it happened, it was an absolute accident. The reaction I got was so freaking disturbing it put me well off trying that again... ever. It's not nice going for a little shopping of basic resources and tools of the (killing) trade with a smile on your face only to end up getting cursed to bits in glorious surround sound, and then finding your friendly deadface lying on the floor, being gnawed on by rats. Shop's closed. Come back next life. That's bound to make one feel bad, methinks.
Yeah, even though Dark Souls somehow ended up being favoured by me, it has to be said that Demon's Souls is significantly more harsh... and it's got way more of that Ctulhu zinging factor, really making you wish you'd be a better, calmer, more intelligent and more patient person. If Dark Souls put you in your place in a manner you feel is demeaning, Demon's Souls is bound to see you getting institutionalized. I know I couldn't believe some of the more or less elegant solutions to problems that absolutely destroyed me, because I plain didn't get it or was unable to think outside the box when it really mattered. After Demon's Souls, Dark Souls felt like a dumbed down, merry-go-depressed sequel to Tomb Raider.
Oh, I just caught myself exaggerating a bit there. Just a bit, though.