As others have noted, it's a dick move. I think everyone kind of knows this. As Solo said "if you have to ask ..." then you know it's a problem.
Save or Dies are dangerously close to dick moves out of the gate. But, they are a part of the game. A save or die that targets the entire group is much, much worse. In general, any save or die should give you pause, a group-wide one should give you pause again. This is particularly important to the extent the PCs have no way of responding to it. That is, at lower levels such effects are particularly bad b/c PCs have few resources to avoid them (save rerolls, death wards, etc.) or mitigate them (revivify, resurrection, etc.).
Further, if you've got a bunch of non-optimized PCs in the party -- and especially if some of the PCs haven't really dived into the metagame of iterative proofing and so forth -- then save or dies are more dickish.
More generally, you should look at the relative firepower you as DM are putting out there on the battlefield. Setting aside bonkers MM2 for a second, you're using a level of firepower the PCs can't get access to for another 6 or so levels. That's not a good sign. The odds that they'd have the tools to begin to adequately respond, and not just be nuked by it, are very slim. There are tons of exceptions to that, but it's a good rule of thumb.
