::Firstly, making up non existent words such as death crime or death defence to argue that death penalty is inappropriate word is spurious. In fact you are somewhat making my point.  You say capital punishment is tied to capital crime. "Capital" denote ultimate or prime or ultimate, making reference to severity or significance. So it can refer to a city (London) as well as crime or penalty. "Death" is a matter of fact reference to the content. For this reason, death city makes no sense. In reverse, death penalty is more matter of fact (NPOV) reference than capital punishment. Moreover, your claim that that "We (American?) uniformely describe crimes elibigle for capital punishment as capital crimes". A simple google search shows that the claim of uniformality is not sustainable even in U.S.A. 

 ::Secondly, your made a claim in term of general usage. Therefore, google search with few millions sample is infintely more authoritative source than your sample (few entries dictionaries which subsequently equate DP and CP anyway). Even if we somewhat restrict ourselves to academic reference (which include legal dictionary or encycropedia), google scholar search, which make comprehensive search of academic papers, shows that death penalty is more common reference. You can also make few other type of search. Try using google news, which makes comprehensive reference to journalism. It shows overwhelming usage of death penalty over capital punishment. Or try using ask.com search which uses subject-specific link popularity to compute "authoritativeness" of a search result. Again it shows that death penalty is more common reference.  Most top search for DP are from U.S as well. Capital punishment may be more common usage in sentencing which is a specific legal procedure in jurisprudence (which explain legal dictionary reference). This restcited context cannot be used to argue for commonality of usage of capital punishment both in public, journalism and academic world. This is another indication that DP is more NPOV than CP. Anyway, if you want to use few dictionaries entry as your evidence, feel free to do so. We appear to be heading to arbitration process. Consensus is prefered but not to the extent of filbuster. Ultimately, I don't have to convince the opposing advocate(s). I only have to convince the jury (arbitrator) that my case is more NPOV than yours.