At the time I'm sitting down to write this, the Buffalo Sabres have lost an astonishing 14 games in a row, have a goal differential 34 goals worse than any other team in the league, and have a shot attempt ratio of just 37.2%! To put that last number into perspective, the lowest Corsi that any NHL team has had since 2007-08 (when the stat first became available) is the 2013-14 Toronto Maple Leafs with 42.8%. So the Sabres are bad. Terrible. Implausible. But just how bad, precisely?
There are probably a lot of ways that you could attempt to answer this question, but I'm going to use goal differential. We could just rank teams historically by goal differential, but that presents a problem: there are a lot fewer goals scored these days than there used to be. We need to account for era, so I've adjusted goal differentials based on a league average of 6 goals per game. I'm also limiting the results here to seasons since 1991-92, when the NHL added the San Jose Sharks, followed the next season by the Ottawa Senators and Tampa Bay Lightning. This ensures that we're still looking at teams that are at least relatively modern.
The goal differential data that I've used comes from Hockey Reference (which kindly added goal differential as a searchable column at my request), and the average number of goals scored per game comes from Quant Hockey. This list includes every team since 1991-92 that had a goal differential of at least -100, plus this year's Sabres. And now - data!
[GD - actual goal differential; NGD - normalised goal differential; GD/game - normalised goal differential per game played]
So the Sabres come in second-worst here, but it's worse for them than it looks. All of the teams around them (Ottawa, San Jose, Atlanta, and Ottawa again) were in either their first or second year of existence, and so we'd expect them to be terrible because they were the product of an expansion draft. Indeed, this list is made up largely of expansion teams. Also, Tampa were terrible in the late 90s, despite having been in the league for several seasons at that point.
So this season's Buffalo Sabres are the worst non-expansion team in modern NHL history, and by quite a wide margin. The next worst established franchise is the 1997-98 Tampa Bay Lightning, who were nearly a third of a goal per game better!
An interesting side-note: the only other non-expansion or near-expansion team on this list (other than Tampa and Buffalo) is the 2003-04 Pittsburgh Penguins, who drafted Sidney Crosby the next season following the lockout. The Sabres this year are the front-runner for Connor McDavid, who may very well be the next player as talented as Sidney Crosby.
So, in conclusion, yes, the Sabres are bad. But they're not just any variety of bad, they're historically bad.