Um...er...because he's a snakehead?
I've only had experience with one species of snakehead (micropeltes) but when I was a kid I worked part time in pet stores and had a lot of experience with that species. They were available everywhere, cheaply and in huge numbers. Very cool fish for a species tank, but a bad choice for the "just one more fish..." crowd.
The micropeltes that I owned, and the many purchased by regular customers who were not too embarrassed to tell the truth, all fell into two categories: those who attacked and destroyed any other fish they saw immediately...and those who did it eventually.