"And for that offense. Immediately we do exile him hence.I have an interest in your hate’s proceeding.My blood for your rude brawls doth lie a-bleeding;But I’ll amerce you with so strong a fine. That you shall all repent the loss of mine."
They deserve death to feel what its like to lose a loved one and that he needs to be killed in a painful way.
AS Juliet is drinking the poison to fall into a deep sleep she thinks shes seeing the ghost of Tybalt who she dearly misses.
"O, look! Methinks I see my cousin's ghost. Seeking out Romeo, that did spit his body. Upon a rapier's point. Stay, Tybalt, stay!Romeo, Romeo, Romeo, I drink to thee!" [She falls upon her bed, within the curtains.]
"Yea, noise? Then I'll be brief. O happy dagger!" [Snatches Romeo's dagger.] "This is thy sheath; there rust, and let me die." [She stabs herself and falls.]
If she cant be with the one that she loves then she sees no point in living her life.