Why can't they continue doing other things that don't involve pain or murder?

They do. There are many high functioning psychopaths in the community, they are often very successful in their careers and this should not be surprising given that research has established that workplace bullying pays off far more than being kindhearted to people does (I believe an article about this citing said research reached Reddit's front page last year). Psychopaths are often incorrectly perceived as being physically violent offenders because the vast majority of those who are diagnosed with Psychopathy Syndrome were diagnosed by clinical/forensic psychologists using the PCL-R in prison. After all, if you're a successful psychopath running a large company or in another position where you have a long way to fall from... why would you want a formal diagnosis?

Personally, I've been in and out of therapy for a number of years and whilst a formal diagnosis can't be made (I personally didn't want one, for obvious reasons and in any case there is an understandably strict procedure that must be followed and even then only in certain circumstances, for the PCL-R to be applied) a well respected therapist believed that I would have a moderately high "score" on the PCL-R and that I'm an example of a high functioning psychopath. Obviously, I had to open the doorway for that conversation because if they raised that with me and I filed a complaint, they could be in some deep trouble.

My motivation for going to a lot of effort to keep myself on the rails is basically that I don't want to end up in prison. I'm in full-time study now, I do MMA training and write/record a lot of music and maintain a very full life like this because if I get bored, that's when I start conjuring up plans to screw people over for my own gain or engineering scenarios that land other people in trouble, simply for that sensation of "duping delight". I also use the internet and its relative anonymity as a playground to toy with the emotions of others and cause a bit of trouble for my own amusement (my comment history here on Reddit is testament to that) because there will occasionally be times when I can't deny the impulses to be cruel and it's better (for me) that I give into them on the internet instead of offline where there will definitely be serious consequences for me.