what is Capital punishment? Capital punishment is the death penalty. It is used today and was used in ancinet times to punish a variety of offences.
This part weakens the whole thing, because it makes it seem like you are going to be giving a definition essay. When you argue against capital punishment, don't stop to define it. Make your argument.
You seem to be talking about 2 different things. The history of it, and the appropriateness of it.
I think you are going to have to give your history of the death penalty as a presentation showing why it should be banned, if that is your argument.
I think you should search your school's library database for articles with the keywords:
"capital punishment" moral
or
"capital punishment" ethical