In order to justify Romeo’s actions, we need to prove that his decisions were reasonable within the set of circumstances he found himself in. While he makes several decisions that could appear seemingly wrong, they could be justified.
Even after finding out that Juliet is a Capulet, in other words his sworn enemy, he continues to pursue her. While this might seem like the wrong decision to make, it could be justified by his intense passion and love for her. For example, upon seeing Juliet at the Capulet party, he says (Act I, Scene V):
O, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiop's ear—
Beauty too rich for use, for earth too dear.
So shows a snowy dove trooping with crows
As yonder lady o'er her fellows shows.
The measure done, I'll watch her place of stand
And, touching hers, make blessèd my rude hand.
Did my heart love till now? Forswear it, sight,
For I ne'er saw true beauty till this night.
As the party ends, Romeo cannot stay away from Juliet and he climbs her balcony. Throughout this exchange, Romeo continues to speak religiously of Juliet, suggesting that he is truly in love with her.
Another decision that Romeo makes that could possibly be justified is that he kills Tybalt. Tybalt challenges Romeo to a duel, but Romeo attempts to hold in his anger since he just got married to Juliet (Act III, Scene I):
I do protest I never injured thee
But love thee better than thou canst devise
Till thou shalt know the reason of my love.
And so, good Capulet, which name I tender
As dearly as my own, be satisfied.
As the duel begins, Tybalt kills Mercutio, and this is when Romeo cannot be peaceful anymore. Because Tybalt killed his dear friend Mercutio, Romeo’s actions of killing Tybalt could be justified (Act III, Scene I):
Alive in triumph, and Mercutio slain!
Away to heaven, respective lenity,
And fire-eyed fury be my conduct now.—
Now, Tybalt, take the 'villain' back again
That late thou gavest me, for Mercutio's soul
Is but a little way above our heads,
Staying for thine to keep him company.
Either thou or I, or both, must go with him.
Interestingly, Romeo blames his actions on Juliet’s beauty, not necessarily his own decisions (Act III, Scene I):
This gentleman, the Prince's near ally,
My very friend, hath got his mortal hurt
In my behalf. My reputation stained
With Tybalt's slander—Tybalt, that an hour
Hath been my cousin! O sweet Juliet,
Thy beauty hath made me effeminate
And in my temper softened valor's steel!
Another fatal decision that Romeo makes is to buy poison from a poor apothecary in order to commit suicide (Act V, Scene I):
Come hither, man. I see that thou art poor.
Hold, there is forty ducats. Let me have
A dram of poison, such soon-speeding gear
As will disperse itself through all the veins,
That the life-weary taker may fall dead,
And that the trunk may be discharged of breath
As violently as hasty powder fired
Doth hurry from the fatal cannon's womb.
This decision can be justified, again, using the relationship between Romeo and Juliet. Romeo has been misinformed that Juliet is dead; he therefore sees no point in living and wants to commit suicide so that he can be with Juliet forever.
Before killing himself, Romeo makes one last fatal decision: he kills Paris in the Capulet tomb. In Act V, Scene III, Paris was preventing Romeo from entering the tomb, blocking Romeo from carrying out his suicide. At this point, Romeo still believes that Juliet is dead and he wants to be with her. Therefore, he kills Paris so that he can carry out his plan.