How are you going to put Tybalt on trial? He's in the tomb. Hum...
Anyway, both of these characters contribute to the age-old feud for which no one remembers the cause. Of course, Juliet lives in a patriarchal (male run society as opposed to matriarchal--female run) society. The men rule the roost. Women obey their fathers and brothers (and other male relatives) until such time they are married, then they obey their husbands, etc.
Tybalt, then, can bear the brunt of the blame since he is more hot-headed and passionate for the feud than most other characters. He was ready to take Romeo down at the masquerade, and then after him from scene to scene up to his death. He also kills Mercutio, who is Romeo's true friend. Romeo responds (as anyone who is egged on to fight will) by challenging Tybalt despite the Prince's edict. Tybalt just kept picking at the scab...he never let the sore heal properly. This is a good point to bring up in your trial.
A case can be made that the only people who caused Romeo and Juliet's deaths are the two of them. Another case can be made that the Nurse and Friar are to blame for the young lovers' deaths. In fact, there is a more direct connection between the two counselors (one who helped Juliet marry a sworn enemy without telling her mother and one who not only conducted the marriage but also contrived to allow Juliet to escape her parents, allowing them to think she was dead). It might make an interesting argument that Tybalt and Lady Capulet may have had a hand in the deaths but that others were more to blame. In the end, it is the patriarchal feud which is to blame, and that means the two Lords are on the same list as Friar and the Nurse. All four of those names are higher on the blame list than Tybalt and Lady Capulet.
With both of them, you can just say that they are forced to do what they do by the demands of a patriarchal society. Lady Capulet is not at fault -- she is just doing what her husband says. In a patriarchal society, she has little choice. Tybalt is just defending the honor and virtue of his kinswoman. In a patriarchal society, a woman's chastity and her obedience to her father (when she is not yet married) are of paramount importance. He is simply trying to prevent an outsider (Romeo) from potentially taking away Juliet's chastity and making/helping her disobey her father.