I think there are two possible ways of interpreting this question. On one level, you can ask why killing Humbaba is important to Gilgamesh and Enkidu. At the same time, however, you can also ask how this action is reflected within the themes and structure of the Epic of Gilgamesh. In this case, the question would be: why is this decision important to the poem itself?
I think other contributors have already addressed the first of these questions (ultimately, for Gilgamesh and Enkidu, I think, the killing of Humbaba is largely about pursuing glory for themselves). With that in mind, I am focusing on the second of these questions.
What's important to remember is that the killing of Humbaba is actually an affront to the god Enlil. From that perspective, this decision can be understood as an example of misjudgment and recklessness on the part of Gilgamesh and Enkidu. Later, Gilgamesh and Enkidu will make further enemies among the gods after Gilgamesh spurns Ishtar and the two heroes kill the...
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