There's a whole procedure for this in the multiplayer section of the comp rules. The big take-away is this: as soon as you die, all your stuff leaves the game -- then, after that, if you would still control anything (e.g. a permanent you stole from someone else), that thing is exiled; any triggered abilities you would own don't occur.
800.4. Unlike two-player games, multiplayer games can continue after one or more players have left the game.
800.4a. When a player leaves the game, all objects (see rule 109) owned by that player leave the
game, any effects which give that player control of any objects or players end, and all spells and
abilities controlled by that player on the stack cease to exist. Then, if there are any objects still
controlled by that player, those objects are exiled. This is not a state-based action. It happens as
soon as the player leaves the game. If the player who left the game had priority at the time he or
she left, priority passes to the next player in turn order who’s still in the game.
Example: Alex casts Control Magic, an Aura that reads, “You control enchanted
creature,” on Bianca’s Wall of Wood. If Alex leaves the game, so does Control Magic,
and Wall of Wood reverts to Bianca’s control. If, instead, Bianca leaves the game, so
does Wall of Wood, and Control Magic is put into Alex’s graveyard.
Example: Alex casts Threaten, which reads, in part, “Untap target creature and gain
control of it until end of turn,” targeting Bianca’s Wall of Wood. If Alex leaves the
game, Threaten’s change-of-control effect ends and Wall of Wood reverts to Bianca’s
Example: Alex casts Bribery, which reads, “Search target opponent’s library for a
creature card and put that card onto the battlefield under your control. Then that player
shuffles his or her library,” targeting Bianca. Alex puts Wall of Wood onto the
battlefield from Bianca’s library. If Bianca leaves the game, Wall of Wood also leaves
the game. If, instead, Alex leaves the game, Wall of Wood is exiled.
Example: Alex controls Genesis Chamber, which reads, “Whenever a nontoken creature
enters the battlefield, if Genesis Chamber is untapped, that creature’s controller puts a
1/1 colorless Myr artifact creature token onto the battlefield.” If Alex leaves the game,
all such Myr tokens that entered the battlefield under Alex’s control leave the game, and
all such Myr tokens that entered the battlefield under any other player’s control remain
in the game.
800.4b. If an object would change to the control of a player who has left the game, it doesn’t. If a
token would enter the battlefield under the control of a player who has left the game, no token is
created. If a player would be controlled by a player who has left the game, he or she isn’t.
800.4c If an object that would be owned by a player who has left the game would be created in any
zone, it isn’t created. If a triggered ability that would be controlled by a player who has left the
game would be put onto the stack, it isn’t put on the stack.
Example: Astral Slide is an enchantment that reads, “Whenever a player cycles a card,
you may exile target creature. If you do, return that creature to the battlefield under its
owner’s control at the beginning of the next end step.” During Alex’s turn, Bianca uses
Astral Slide’s ability to exile Alex’s Hypnotic Specter. Before the end of that turn,
Bianca leaves the game. At the beginning of the end step, the delayed triggered ability
generated by Astral Slide that would return Hypnotic Specter to play triggers, but it isn’t
put on the stack. Hypnotic Specter never returns to the battlefield.
800.4d. If combat damage would be assigned to a player who has left the game, that damage simply isn’t assigned.
800.4e. If an object requires a player who has left the game to make a choice, the controller of the object chooses another player to make
that choice. If the original choice was to be made by an opponent
of the controller of the object, that player chooses another opponent
800.4f. If an effect requires information about a specific player, the effect uses the current information about that player if he or she
is still in the game; otherwise, the effect uses the last known
information about that player before he or she left the game.
800.4g. If a player leaves the game during his or her turn, that turn continues to its completion without an active player. If the active
player would receive priority, instead the next player in turn
order receives priority, or the top object on the stack resolves, or
the phase or step ends, whichever is appropriate.
800.4h. When a player leaves the game, objects that player owns in the ante zone do not leave the game. This is an exception to rule 800.4a. See rule 407, “Ante.”
800.4i. In a Planechase game, if the player designated as the planar controller would leave the game, instead the next player in turn
order that wouldn’t leave the game becomes the planar controller, then the old planar controller leaves the game. See rule 309.5.
So, in your example, it's actually pretty straightforward:
Alex's Massacre Wurm is on the stack. Bianca flashes in her own Massacre Wurm (thanks to Vedalken Orrery or whatever). Alex has no further response, so we resolve Bianca's Massacre Wurm. It comes into play and kills Alex's creatures, putting him at below 0 life.
When Alex loses the game, his Massacre Wurm is still on the stack. It leaves the game directly from the stack, without ever resolving. Poof!
When Alex loses the game, his Pacifism also leaves the game. Its effect on the creature it was enchanting is gone, just like if it had been removed any other way.
Here's the tricky part about these rules... Let's say Alex also had an Oblivion Ring that he had previously used to exile Bianca's Liliana of the Veil. The O-Ring leaves the game, but does not trigger any effects when it does. Including its own. So Bianca's Liliana remains exiled forever.
Also, note that, assuming it was Alex's turn when he died, the other players play out the rest of the phases without an active player. If Bianca had some mana open and she wanted to cast another spell before her own turn, for instance, she could still do it on now-dead Alex's end step.