There are so many aspects to the death penalty that you could write about. To get to 15 pages, you should pick a few of these aspects and start researching them. The link below has a large number of essays on various aspects of capital punishment.
Some issues you might address:
- Is the death penalty administered fairly? (Or can it be administered fairly?) There are real issues about racial bias in the death penalty and about the possibility that people who cannot afford good lawyers are more likely to get the death penalty than those who can.
- What sorts of justifications are there for the death penalty?
- Which of these justifications can be supported with evidence? A major example here would be the issue of whether the death penalty actually deters crime -- does it reduce the rate of homicide, for example.
I believe that you could get more than 15 pages out of these three questions.