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Just to nitpick, we technically don't see Louis being killed in the movie. We see Rachel pick up a knife while they're kissing and then it smashcuts to black as he shouts "no". So yeah, the implication is that he gets stabbed to death by Rachel but it's technically still up in the air.

As for the novel...I think by the end Louis probably almost wants to die. Or at least he doesn't much care about surviving. He's watched his three year old son get run over by a truck (and the descriptions indicate that it was an utterly brutal death), he's then lost his father-figure and wife and had to kill his three year old son again with the disturbing added implication that Gage was more like his former living self just before he died the second time, and if the narration is anything to go by he's even aware that he's going completely insane from all the trauma that's been heaped on him in a relatively short period of time. I don't even think it's a case of "not learning his lesson" by the time he resurrects Rachel, it's just a combination of the Wendigo's influence, his own loss of sanity and a growing lack of concern for his own continued survival (plus quite probably a healthy dose of guilt thinking that it's fundamentally his fault that Rachel was killed due to him bringing Gage back). I'm fairly sure he knows exactly how Rachel is going to be when she comes back, he just simply doesn't care anymore by that point. It's kind of ironic that the one member of the family who initially seems to have a healthy outlook on death and how to deal with it at the start is the one who ultimately ends up with the least healthy outlook on death and how to deal with it by the end.

As for Elle, I'm fairly sure I read somewhere that Stephen King said that after the end, she is brought up by her grandparents and eventually grows up into a fairly well-adjusted adult but she continues to have nightmares about the pet cemetery.