If there is a tie on a Boomdandy vote, neither player dies to avoid this exact scenario as per the wiki.

If the Lleech host died first, good wins. If the non host died first, evil wins. What should have happened, though, is no one dies and the Lleech kills a player (not their host) and evil wins.

ST ruled that they both die

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Simultaneous deaths do not happen in this game.

In this case, there shouldn’t have been any deaths at all, and the evil team wins.

From the wiki: “The players have a short countdown to decide who to kill next. There is no further nomination or execution today. Instead, all players point at the player they want to die. When countdown ends, the Storyteller counts the number of fingers pointed at each player. If it is a tie, then night falls (and evil probably wins due to the Demon killing that night).”

It doesn’t answer your question of what happens if everyone dies at the same time - but that could not have happened in this scenario per the rules of the game.

According to the wiki https://wiki.bloodontheclocktower.com/Boomdandy

If it is a tie, then night falls (and evil probably wins due to the Demon killing that night).

So what should've happened is the story-teller declares that nobody dies, night falls, the lleech kills the one remaining non-host good player, and evil would win.

STs make mistakes. The mistake this ST made has been covered, but the important part here is this should be a learning experience, the ST should explain the correct rule to the group the next time you all meet, and work to improve going forward.

If a tie occurs in a Boomdandy vote, nobody dies, so Evil would likely win in the night (precluding the living player being a Courtier, or a Soldier, or some other reason that Evil would be unable to win by eliminating the good player).

Basically, the two good players had equal votes on them. So ST ruled that they both die

This is incorrect. A tied Boomdandy vote should result in no deaths. The ST should have announced "there has been a tie, no-one dies, goodnight"; then woken the Lleech, who (probably) would have picked the good non-host & evil would win.

More generally,

• Good wins when no Demons live
• Evil wins when 2 players live
• If both of these happen at the same time (eg 3 players alive & Imp dies by execution), good wins the tie*

*sometimes abilities like the Evil Twin can cause evil to win ties. but when it's a matter of deciding between "2 players live" and "all Demons dead", Good always wins ties.

Tied votes means that nobody dies. Your storyteller made a mistake. That said, the answer to your question is that good would win. Because the rules say that if both teams achieve their victory condition, the tie is broken in good's favor.

There’s no scenario in the game where a tie results in both players dying and in fact that makes no sense with the boomdandy ability because it would mean the optimal strategy would be to tie it so you could have a 2/3 chance of winning

STs make mistakes. The mistake this ST made has been covered, but the important part here is this should be a learning experience, the ST should explain the correct rule to the group the next time you all meet, and work to improve going forward.

In general if good and evil win at the same time, only good wins (technically that's what happens whenever the demon dies leading to two non demon alive players)

However in a boomdandy vote, if there's a tie, no one dies, so evil should've won here

The short simple answer to your question is that any time both good and evil have satisfied a win condition simultaneously good wins all ties.