I've been a sadist for as long as I can remember

So basically what you're arguing is that everything that makes anyone different, sexually or otherwise, can be seen as a mental disorder. The legality of S&M is only an issue if they are seeking sexual gratification without consent, which would be considered, at the very least, sexual harrasment. However, to say that practitioners of S&M are more likely to take their desires out on non-consenting partners is ridiculous and offensive. It's the equivalent of thinking that all Gay people will try and have sex with you just because you are the same sex as them.

If finding sexual gratification from giving or receiving pain does not negatively impact your personal or professional life, then why should it still be considered a "problem"?

I hate to burst your hetero-normative bubble, but many people experiment sexually. The reason why much of this experimentation is considered "taboo" is because of conservative people like you who want to classify everything outside the norm as a "mental abnormality".

I'm not saying they WILL try and take their desires out on non-consenting partners, just that their DESIRE to cause pain is unlikely to be limited only to consenting partners. The gay guys I know do sometimes lust after straight guys, and I've definitely found gay guys sexually attractive before. You don't have to actually do anything illegal for your thoughts to be considered unhealthy/psychologically abnormal.

An extreme example: a person can have psychopathic tendencies without actually going ahead and actually killing a whole bunch of people.

I'm going to go ahead and tell you that I've experienced S&M before, in a submissive capacity. It turns me on, but I'm not exactly sure its mentally healthy for me to get off on pain and humiliation. I've also noticed that the worse I am feeling about myself the more thats the sort of sexual experience I seek out, almost like self harming in some ways. I can't be the only person who feels the same way, and its not unreasonable to ask whether apparent consent is enough in that situation.

I think everyone has done these things. Got any more recent examples which aren't in young childhood?

Only a few months ago me and a female friend were play fighting, my sadistic urges took over and I started being very rough with her and ended up making her cry. Obviously things like that make me ponder the whole thing..

and so how did you feel when you made her cry? Did you have any regret/guilt, or was the fact she was upset purely a turn-on?

You might want to see a psychiatrist. That isn't meant to be an insult, but that kind of behaviour isn't normal if you didn't feel guilty and took pleasure out of hurting other children when you were younger and still enjoy hurting people today as an adult.

Yes me too, oh wait, No.

I didn't feel any guilt at all because I thought that she secretly liked it, and to be fair, she probably did. I think that might be a worthy point actually, in my later teenage years I've been exposed to girls who like being submissive and are into being tied up, gently bitten, being teased, being treated rough, being pounded hard in the bedroom, having their hair pulled, etc which has only exacerbated the problem. Are you like me?