Sociopath or schizophrenic?

I think that the movie is confusing unintentionally becuase some elements of it are poorly done and certain people are attempting to explain what is going on by using concocted theorys that are far more advanced than anything the filmakers intended.

As for Simon's mental state, I think that he was an inadequately developed and silly character and to lood for a legit explanation for his actions grounded in reality is to offer this film more credibility than it has earned

neither, he is completely sane(something else is wrong with him). Notice that he is drawing the same picturers throught the movie.

Do you mean Anti-Social Personality Disorder or do you mean Psychotic, when you say "sociopath"?

I don't think the two diseases you mentioned (AntiSocial Personality Disorder or "Sociopathy" and Schizophrenia) are interchangable the way you think they are. They don't really share much (if any) diagnostic criteria, and one is a "personality" disorder while Schizophrenia is an affective disorder. Schizophrenia is more charatceriszed by hallucinations (visual and auditory) and fairly fragmented, disorganized thought processes, bizarre and delusional thoughts and behaviors; whereas ASPD is characterized by flagrant and perversive disregard and violation of the rights of others, blaming others, lack of morals and guilt, low tolorence to avodi anger and violence.

I don't think we really learned enough about the character to tell if he is Schizophrenic, although the co-morbidity of another disorder WITH Schizophrenia is usually very high, so it's possible he has both.

Oder: Wie ich aufhören, sich Sorgen und Liebe die Bombe gelernt!

I think neither... remember how now and then, other people besides him "see" those bloody ghostly people behind him....I take it to mean that there's a supernatual "demon" element that's making him keep on committing more and more murders. That woman with the red colored glasses in slippers in the beginning, even both doctors, they all see those ghastly looking "visitors" as some point in the movie, so they seem to be real. Unless that's what the lady meant when she screamed at the woman doctor "He'll get inside your head too!"...so are these demon visitors that tell him to kill real, or inside his/their head? Hmmm...good question.
Now, whether what attracted them to him in the first place and/or brought them on, and started the first killing, that could mental illness brought on by his traumatic childhood.