Why should a proficient negotiator be a pacifist? If anything, I'd argue that it actually hurts you in many cases. Violence is just one of many ways of resolving interpersonal conflict, and taking the option off the table only serves to tie your hands.
It's been my experience that being ready, willing, and able to kill everyone you're negotiating with can come in very handy, especially when dealing with bullies, thugs, and gangers. You have to dial it down, or shut that off completely with some people, but I'd never give up being able to turn it back on.
Oh, and I love gaming horror stories. What's this pacifist adept trainwreck tale of woe?