"出外猎取人头" can be translated into English as "go out to hunt for heads" or, in a more figurative or game-related context (like in video games), "go out to farm kills" (though "farm kills" is more specific to gaming terminology).
If you're looking for a more general or everyday expression that captures the idea of seeking out and achieving a goal (in this case, a violent or aggressive one, though the literal translation is quite direct), you might adjust the phrasing slightly depending on the context. For example:
"Go out to seek heads" (a more literal but slightly archaic or poetic translation)
"Venture out to take lives" (a more dramatic and violent interpretation)
However, "go out to hunt for heads" is a straightforward and somewhat literal translation that conveys the basic meaning of the original phrase.