In a general context we would ask this using the infinitive (either marked with to or unmarked), since that's generally regarded as the 'base' form of a verb.
What is the English translation of 'ermorden'? Does it mean "to kill"? OR
What is the English translation of 'ermorden'? Does it mean "kill"? †
In some specific contexts, however, things get a little more complicated, because both the infinitive (to) kill and the gerund-participle killing may be appropriate translation of ermorden employed as a nominal designating the act or practice of killing.
"Kleine Kinder ermorden ist schließlich das schlimmste und gemeinste." How should I translate ermorden here? Would "to kill" or "killing" express the sense more accurately?
†And in a general context the answer to both of these is "Probably not"—ermorden will usually call for a stronger translation as murder or in some circumstances assassinate—or as a nominal, assassination.