The main reason why Juliet is afraid Romeo will be murdered of course stems from the fact that their two families are engaged in a longstanding feud. As a result of the feud, both families feel a great deal of animosity towards each other, which leads to fighting and even death. Since all of the instances in which Juliet expresses fear of Romeo being murdered are similar and for the same general reason, below is an idea to help get you started.
One place where Juliet expresses concern that Romeo might be murdered is when, after crashing the Capulet's ball, Romeo sneaks into their garden hoping to get another glimpse of Juliet. When she sees him, the first thing she asks him is how he got into the garden and why considering that her family members will kill him if they see him, as we see in her lines:
How cam'st thou hither, tell me, and wherefore?
The orchard walls are high and hard to climb,
And the place death, considering who thou art,
If any of my kinsmen find thee here. (II.ii.66-69)
Since Juliet says that her "kinsmen" will kill him should they find him in the garden, it's pretty clear that Juliet is afraid Romeo will be murdered by her family members. She is afraid he will be murdered by the Capulets because their two families are feuding against each other, and he is trespassing on the Capulets, which puts him in grave danger.