I don’t think it’s an accident that there is a conflict between these two customs: guest right exists somewhat to create a systemic dis-incentive to kill hostages. And that’s not a bad thing, because the point of hostage-taking (and especially true with hostage-exchanges) is to create an alternative to wiping out your enemies root and branch, similar to how the custom of ransoms is there to encourage people to take defeated nobles prisoner instead of murdering them for the rings on their fingers.

However, there are cases where you need to execute a hostage. My guess is that the cultural circle is squared through giving hostages guest gifts:

“The Freys came here by sea. They have no horses with them, so I shall present each of them with a palfrey as a guest gift. Do hosts still give guest gifts in the south?“

“Some do, my lord. On the day their guest departs.”

“Perhaps you understand, then.“ (ADWD)

Thus, the hostage is no longer a guest and can be executed without violating the taboo.