just my non-researched opinion, but i would expect the results to come out as you said they did. i always thought the issue to be less of a stance on whether to implement death (or not), and more of a stance on the role of the law. for example, social liberals, may advocate less intervention of the the law: the law cannot/should not make a decision on life/death, hence pro-choice and against death penalty. on the other hand, social conservatives would advocate the law to intervene when it seems necessary, hence pro-life and for death penalty.
...less to do with a stance on death and more to do with a stance on the role of the law.
im sure there are moral components too, but youre right, they way the tendencies fall, they dont seem to align so obviously on a moral plane.
...interesting paper topic.