I happened to notice while looking at the Steam badges for Torchlight that more people have the badge for beating the final boss (Beast Slayer I, 17.8%) than for beating the penultimate boss (Only a Master of Evil, 16.0%.) This seems a fairly substantial discrepancy.
The only explanation I can think of is that some of the badges were introduced later? Or perhaps there's a way to merely avoid Alric instead of fighting him.
Or, I guess, the most likely explanation is just that Steam achievements are buggy. :)