The two images below were taken immediately after a single match, while playing as Sedna. Prior to this match, my achievments had been wiped clean (for Sedna, at least) as per the current favour/acheivement bug, so (I assume) I was working from a clean slate when this happened. Take note of the "Sedna the Exterminator" achievement in the first image. Kill 25 Demigods in a single match. Simple enough... When dealing with bots, at least. 

The following image was taken at the same time. Note the fact that the requirements for "Sedna the Butcher" haven't been reached.

So. I've killed 25 Demigods in a single match, but still haven't managed to... Uhh... Kill 25 Demigods. Wait, what?

Unfortunately I didn't bother to check my killcount at the conclusion of the previous match, but It's my guess that the killcounter for Exterminator is tallying solo kills & assisted kills, while Butcher is working solely on solo (non-assisted) kills. Not a huge issue, but I thought I'd better make note of this, just in case this ISN'T the way that the devs intended these achievements to function.