Have another beloved character kill them. Better be a close friend to the character, one that absolutely no one suspect, one that everyone can't help but love. One who is always enthusiastic and encourages your character.
THEN:

• Reveal the truth only when it's too late and the killer escapes.

• Have a cowardly character know the truth and never tell anyone.

• Have another character discover the truth but die before telling anyone else.

• Have the said character^ not actually die, but go through something that they forget the friend of the deceased is actually an asshole.