What ideas in Romeo and Juliet support the theme of death caused by human action (besides Mercutio's and Tybalt's deaths)?

You have identified the first two deaths that would jump into my head.  The connection between human actions and those deaths is fairly straightforward.  

For other deaths connected to human actions, I would use Romeo and Juliet.  Had they not fallen in love with each other and gotten married, they would not have been willing to die for each other.  Perhaps that seems like a stretch, so I would also focus on the actions...

You have identified the first two deaths that would jump into my head.  The connection between human actions and those deaths is fairly straightforward.  

For other deaths connected to human actions, I would use Romeo and Juliet.  Had they not fallen in love with each other and gotten married, they would not have been willing to die for each other.  Perhaps that seems like a stretch, so I would also focus on the actions of Friar Lawrence.  I have always thought that he was a little quick to want to have Romeo and Juliet get married.  He knows that the feud between the two families exists, and he hopes that the marriage will patch up the fight.  

For this alliance may so happy prove,
To turn your households' rancour to pure love.

It never really occurs to him that the marriage might actually make things worse.  It's possible that had Friar Lawrence refused to marry Romeo and Juliet that they would not have wound up dead.