What causes Juliet to have so much difficulty finally understanding that Tybalt has been killed by Romeo?
Juliet first hears that Tybalt has been killed from her Nurse and then her mother. She mourns quite loudly for him until she learns that Romeo is the murderer. Her crying immediately stops and she starts to question the Nurse about how she received her information and why Romeo would be involved at all. Romeo had JUST left her bed that morning after they had been secretly married by the friar. She cannot believe someone so in love with her can turn his heart to stone and kill one of her kinsmen so quickly.
As far as she knew, Romeo was off to seek peace between their families and show off their marriage as its first sign. She already believes it was an accident by halting her tears and asking so many questions of the Nurse. When her mother confirms that it was Romeo that killed Tybalt, she is stunned into questioning their love and how it was now doomed by death. She loves him that much that death is the ONLY way she sees out of this mess. She doesn't once question Romeo's loyalty, but constantly questions her family's. The only person in her household that she deems trustworthy is the Nurse who raised her and the childhood friar who married her. Perhaps she witnessed the exchange between her father and Tybalt at the party the night before. Perhaps this lead to her questioning Tybalt's hot temper and wont to fight anyone he deemed the enemy.
Even though she mourns Tybalt, there seems to be a slight sense of understanding that Tybalt brought death upon himself.