I need help giving an example from Romeo and Juliet by William Shakespeare that illustrates these tragic elements: Comic Relief and a Revenge Motive

I think you could make a case for Lord Capulet, at one point in the play, to provide comic relief, and at another point in the play (by a very surprising turn of character) to provide action motivated by revenge.

In Act I Scene v, he's a rather jolly party giver, kidding around with his guests and servants -- even cautioning Tybalt that Romeo (their family's enemy) should be endured at the party for the sake of the festive atmosphere.  This is a man who seems to be harmless and sort of silly.  A good-hearted man who loves his family.

In Act III, Scene V, we are surprised to meet a very different man.  At first, when Capulet enters Juliet's room to receive her "gratitude" for her impending marriage to Paris, he seems, again, light and tender hearted, even making fun of Juliet's tears, presumably to laugh her out of her grief. ("How now, a conduit, girl?  What, still in tears?/Evermore showering?")

Yet, once it is clear that she is, in fact, not only not thankful for his "gift" of Paris as a husband, but even refusing it, he turns on her.  "Hang thee young baggage, disobedient wretch!"  Even condemning her (his act of revenge) to "beg, starve, die in the streets."  It is worth noting that this scene is often played with Capulet taking his revenge out on Juliet with physical violence.  If not an actual violent act, then at least the threat of one -- "My fingers itch."

Romeo and Juliet is often cited as the first play in the English language in which each character speaks in a truly individual rhythm, separate and distinct from the other characters in the play.  And there is certainly nothing generic about Lord Capulet.