Sorry about the confusion, I'm not the best at writing rules clearly and I'll be hiring an editor for future endeavors.
A level 6 cowpoke would instant kill on a roll of 3, 4, 5, or 6. Leveling up increases your instant kill range, not change the number.
Level 1 cowpokes would also instant kill on a roll of 3, the range doesn't really increase until level 4.
Sorry again for any confusion!