I've been trying to get the achievement where you kill someone with the blow torch and have killed maybe a dozen guys with it, and still nothing. I tried just lighting them on fire and standing back to watch, and I've also tried holding the torch on them until they are dead on the ground, still no achievement unlock.
What's the secret? Can you not do it while disguised in the uniform?
the secret is called "UBI" -- you did it right. Just don't kill friendlies, but any red shirt or yellow jacket will do. Set them on fire. :) If you're wearing the privateer costume, you should at least pick a fight before using the blow torch on yellow jackets. ^-^
I'm pretty sure I tried it while in a fight with the yellow guys, the old airport is a nice place to pick fights, as by the time you clear one end, they've respawned at the other end, usually not knowing you are a "bad guy" and you can start all over again. But after a few tries there getting into fights and killing guys with and without the blow torch, I never got the achievement unlock. Guess I'll keep trying.