Where are some instances that we can see the act of betrayal being criticized in Shakespeare's Romeo and Juliet?
One instance of betrayal that we see is the moment Romeo and his friends decide to crash the Capulet's masquerade ball. Wearing masks symbolizes deception, which can also be considered betrayal. Although the incident leads to Romeo meeting Juliet, which can be considered a good thing, the moment of betrayal also leads to a great deal of damage. Tybalt recognizes Romeo's voice at the ball, and feeling dishonored by Romeo's presence in his home, vows to revenge himself on Romeo, as we see in his line, "This intrusion shall, / Now seeming sweet, convert to bitt'rest gall" (I.v.96-97). Tybalt's anger of course leads to Mercutio's death, Tybalt's own death, Romeo's banishment, and eventually, indirectly, Romeo's own death. However, since Tybalt's anger was inspired by Romeo's betrayal through his deceptive mask at the ball, we can see the moment of the ball as being used by Shakespeare to criticize the act of betrayal. All in all, the act of deception, or betrayal, leads to a great deal of damage.
We can also see betrayal in Friar Laurence's activity. Catholic women at that time period were permitted to marry without parental consent at the age of 12 and men at 14, hence, Friar Laurence did not break any ordinances in marrying the couple without parental knowledge. However, Friar Laurence did commit an act of betrayal in faking Juliet's death. Not only that, his plan went terribly awry, again proving that Shakespeare is using Friar Laurence's activity to criticize betrayal. We see Friar Laurence confess to his betrayal when in the final scene he begins to relay his account of what happened with the lines, "And here I stand, both to impeach and purge / Myself condemned and myself excus'd" (V.iii.237-238). He even finishes his account asking the Prince that if he is to blame for any of their deaths, then to please end his life through the Prince's decreed capital punishment, as we see in the lines,
[A]nd if aught in this
Miscarried by my fault, let my old life
Be sacrific'd, some hour before his time,
Unto the rigour of severest law. (V.iii.277-280)
Hence, we see that because even Friar Laurence partially blames what happened on his own betrayal, Shakespeare is using Friar Laurence's actions to criticize the act of betrayal.