There are three EASY and HARD TO SCREW UP ways to kill an animal:
1) Hammer or a bat.. Anything that is a large enough swingable lever + heavy blunt object. A few sharp strikes (one good strike does the job, but a second or third are good in case you hesitated on the first) just to the side or top of the skull, right where the brain is situated. This is my preferred method of dispatching fish that I'm harvesting for food. It's much better than letting them suffocate, or letting them bleed out after slitting the gills (which I also do for best meat quality, but only after dispatching the fish). This gets less suitable as the size of the animal grows, but snakes heads aren't exactly armored and should still work even on very large burms.
2) gun. One large caliber hollow point bullet or shotgun slug to the brain cavity. This is difficult to do on reptiles if you are not versed in how to target the brain. Just ask people who hunt gators or nile crocs how difficult it is. The brain shot on a reptile is just about one the most difficult large game hunting shots that there is. Luckily burms have small heads, at least compared to crocs and gators, so this should be easy with a large caliber round.
3) sharp pointy object to the brain. As morbid as "scrambling the brain" sounds, it works and works fast, but you have to penetrate the skull. This is more physically demanding than the two above methods, so hopefully you are better prepared than needing to do this.
I'm freaking tired of people who don't like messier ways of dispatching animals.
As a society we're obsessed with silent unobservable death. If you've never seen a lethal injection death, you should. You'd quickly change your mind about it being "humane" more than likely. Gas-related death is particularly difficult to do with reptiles. Death by hypoxia or toxic gasses takes much too long with reptiles to be humane. The metabolism is too slow for hypoxia or a toxic gas to kill them quickly.
The only reason to avoid physical trauma for dispatching a creature is for the benefit of the one in charge of how it is performed, and the observers.