Because I can. 90% of the people who said they would kill in any sort of way would in fact have troubles doing it and would be scared afterwards. Your personality can be dark, but taking a life is something very very different and it effects you in many ways, mentally you will never be the same again and if you were there is something wrong with you. The desire to kill from annoyance is a natural human over reaction and not one that is easy to follow through on.
Someone who can kill without the slightest effect on them has likely been in and out of therapy their entire lives, has disattachment issues, and can normally be identified at a very young age. Someone who just feels they wouldn't or has a 'dark' personality does not fit that sterotype or we would see so many more people in institutions these days just as a preventitive measure.
Also to respond to the original question. You can never be FORCED to kill anyone. You have to make a choice and if it's your life vs theirs then you have just chosen your life, which doesn't make you dark in anyway it makes you a regular normal human. Very few would bother with being more noble and giving their own life, but likely the person forcing you is going to kill you both anyway. This topic to me is not needed and sounds like a place for people with 'dark' personalities to get their jollies off and see who the bigger badass is. I feel lucky that none of you are my patients so I can just tell you what I think.