Who wins when the would-be last wizard standing is killed by a reaction?
Based on the rules, Player A should receive the sole LWS token. (For official ruling, see below the break.)
The relevant rule can be found on page 17 (emphasis mine):

When the dust settles and there is but one Wizard alive, the game ends and that Wizard earns a Last Wizard Standing token. In the unlikely event that a Wizard kills himself, leaving no Wizards alive, that Wizard still earns the token. Going out in a blaze of glory is never frowned upon in Epic Spell War!

As you've already noted, in order for a Reaction to happen, the player needs to have died according to the rules. This is confirmed by Matt Hyra from Cryptozoic on Board Game Geek:

Gregg Prendergast: Doers the &quot;reaction&quot; effect only take place at the time your character is killed, or does it take effect any time you are attacked (even if you don't die as a result of the attack)?
Matt Hyra: Only applies if you are killed. If you are not killed, any Reactions in your spell remain face down.

So the order of events would look something like this:

Player A kills Player B
Player B dies
Player A is the last remaining Wizard
Player B has a Reaction that kills Player A
Player A dies
Steps 4 &amp; 5 don't really matter because Player A receives the LWS token even though they are dead


I asked this out on Board Game Geek to get an official answer. From Matt Hyra, Cryptozoic R&amp;D, in response to my question:

The player had to die to trigger the Reaction, so unless the Reaction tells you you are resurrected, the Wizard who killed you gains the LWS token.