Hm.. actually, I don't think they can legally re-instate the death penalty without leaving the EU. (
http://ec.europa.eu/comm/external_re.../adp/index.htm)
But somehow I doubt that reinstating capital punishment was their intention to begin with, since public opinion in Europe (as far as I know) is fiercly against it. It is more likely that this was considered a cheap and easy way of scoring points with their right-wing supporters. It's something see it all the time in The Netherlands. Politicians launch an impossible plan that has no chance of succes whatsoever but is likely to sit well with their supporters. They get a ton of media coverage and can blame the EU when things don't work out. It's a perfect construction.