The argument that sexual sadists are insane psycho killers can also be put onto sexual masochists: they want to be abused and victimized, murdered. It's a load of BS. Just because you like to spank (or be spanked) by your lover in bed does not mean you are a serial killer, or desire any non-consensual hurt on others/yourself.
I can understand why SM is so misunderstood. The media constantly sensationalizes it and (like any subculture or anything not viewed as "socially normal") twists it to its own ends... ratings. Sure, Sadism and Masochism were in previous versions of the DSM, but then... so was homosexuality. As the medical community's views changed about things, the DSM has been modified. It is very important to note that sexual sadism/sexual masochism are only considered mental disorders if the person afflicted has impaired occupational functioning (work, family roles, basic life skills), and/or involve non-consenting partners (or self destructive behaviour).
Your teacher making such broad generalizations speaks to his ignorance and lack of education on the subject, or perhaps he simply cannot fathom why or how anyone could partake of sexual interests that are portrayed so violently in a willing, non-crazy way. It is easy for someone that hasn't educated themselves about it to read such things into terms like "Master" "slave" "Sadist" and "masochist". They can be very "loaded" words.