I think it really comes down to this: Were these 2 normal young people (as normal as they appeared to be?) or was there real pathology brewing? The drugs or acid need not even make an appearance, as I will explain below:
I used to think it was stupid to make the leap to 'pathology brewing'. Until I began to read about Paul Bernardo and Karla Homolka (NOT saying that Knox and Sollecito are anything like these 2, though).
The latter in all aspects appeared normal and well-adjusted: So much so, that when a police composite looked like Bernardo, they still didn't bother to test his DNA. And why? Because he was white, blonde, blue eyed, smiley, polite, soft-spoken, eloquent; he was college educated, came from a good family, and all the same could be said of his young, demur wife. Two pleasant and well-liked people in their 20s, with a profession, a house, lots of love from family and friends.
Why on earth would they be compelled to embark on kidnappings, drugging, raping, sodomizing, strangling, dismembering? And yet they did.......So what does this have to do with Knox and Sollecito? Nothing, except to illustrate that IF there is real pathology, the exterior may hide it so well that no one suspects a thing until the bizarre crime has occurred.
And it doesn't even take drugs or acid for such to occur, if the fuse is lit, and the pathology deeply rooted. The scariest thing is, it was only their 2 personalities fused that caused just the right chemistry for real pathology of the degree and kind they displayed.
Could Knox and Sollecito have been a fatal combination?
Possible? Yes. Plausible? Only if they were lunatics under the skin. All hinges on this. Otherwise, to suspect them is absurd.