While he does look like he could be Fire/Fighting, I am so glad he's not. We've already had too many starters of that typing. Delphox was a relief when I found out she was Fire/Psychic. Same goes for Fire/Dark here.
Also type makes sense if you are
familiar with professional wrestling and its terms.
If you are, it'll look like a heel wrestler, and thus Dark type
If you aren't, it'll look like a wrestler, and thus Fighting type
It's a wrestler, so of course it's going to look like a Fighting type, but since it's a heel and heels are known for using dirty tactics, which is what the Dark type is all about, it's Dark