Why do all these killers need motivation and explanation? You want to scare people, just let them BE the evil, the horror, and all the things that disturb us. Back stories are fine, but I personally find one of the most disturbing elements of any mass murderer being when there is no cut and dry, straight forward reason. Some people just kill. Not for kicks, not because they were abused, not out of revenge. Some kill for reasons personal only to them, and all the hollywood psychoanalysis and Freudian synopsis they try to attach to that can actually diminish the real horror of a killer who kills just because.
I always thought of Freddy Krueger as a sort of Groundskeeper Willy from the Simpsons. Socially detached, awkward, but still deeply entrenched in his job and the school he works for. It also gives Kreuger both the tools and opportunity for his original mortal sins. What if when he molested the kids he gave them ketamine and an lsd concoction to leach out of them whatever their greatest fears were. He feeds on nightmares. So when he returns in whatever supernatural form, there is some reason for his use of the nightmare. Personal interest. Nothing more. His increasing knowledge of what terrifies others, makes him completely sociopathic and detached from life. When nothing scares you anymore, you have to create new terrors to fill that NEED. Stretch the limits of decency and safe viewing here. Don't give us Universal nightmares we all know people are scared of. Give us raw, primal, fears that even the most stoic of us would shudder and beg to be saved from.
Make us uncomfortable. Instead of giving us another horror film, try to give US nightmares.
Oh yeah, and staying "true" to the original line up of cast members is not nearly as important as the script at this point. You have someone who can carry the film on his performance alone now if needed. Just make sure you don't ruin HIS performance by surrounding him by half wit, bubbled headed, Gossip Girl quality teenage actors.