Theoretically, we were the right level and right number of players. As I wrote upthread, the adventure was written for 5E and I converted it to A5E, so maybe that was part of the issue.
However, this is just one example in a long, long series of TPKs. We're talking around a dozen occurrences with more than a score of players since the year 2000. There's a larger root cause than one poorly planned wererat encounter.
So I passed the torch to another DM after this TPK. Already, about two months later, the rest of the group is already wanting me to take back the reigns. Therefore, I'm wanting to work on this TPK thing.
It's a fairly major issue in this particular case, since wererats in A5E are hellaciously punishing compared to 2014 wererats.
I mean, it's right in the stat block that they average 12 damage a hit if they have advantage, and they have 2 attacks (3 if bloodied), and they have pack tactics, which means easy advantage. That tells me that one wererat by itself is easy pickings, but 2 or more is going to be a nightmare for a Tier 1 party. A level 3 character averages in the low 20s for HP, so one wererat can drop a PC from full in one round on decent rolls, and 2 teaming up is almost certainly going to drop a PC barring extremely high defense or extreme bad luck on the GM rolls.
Without having some form of hard CC (not common in Tier 1), or having a lot of ranged magical damage and good kiting (remembering that they have nonmagical weapon resistance, so that 33 HP is actually 66 for most Tier 1 parties), 4 level 3 PCs against 4 wererats is like walking into a Cuisinart.
To me, the core issue is if you can't look at a fairly simple stat block like the wererat, run that simulation in your head and realize that this fight is going to wreck the PCs, you're going to have issues running a combat-heavy game without the crutch of a
perfectly designed adventure (which doesn't exist).
This is especially relevant since the party does not seem to want to play anything close to an old-school style, wherein they "feel out" the encounter and run if its too hard. They're expecting to play in modern neotrad style where encounters are interesting but ultimately winnable challenges.