Explanation of the listed verses
As stated in the comment of @UmH and the context also is clear and easily distinguishable.
They said, "Indeed, we have been sent to a people of criminals, (15:58)
"they" refers to the Angles who were introduced in 15:51 and 15:52 first as his guests. So the angels were sent with orders to save some people as stated in the next verse:
Except the family of Lot; indeed, we will save them all (15:59)
"we" here applies to the angels fulfilling their order of saving the followers of Lot().
The sahih international translation shows whom decreed and gave the orders saying:
Except his wife." Allah decreed that she is of those who remain behind.
So basically it is clear that angels follow orders since Allah informed us about this in the verses:
the angels ... They fear their Lord above them, and they do what they are commanded. (16:49-50)
angels ... they do not disobey Allah in what He commands them but do what they are commanded. (66:6)
So there's no doubt about whom decreed or whom gave the orders! And therefore all other translations use the "We" in capital letters.
Nevertheless if we took the speech in the long conversation of which we displayed some snippets (since the conversation between Ibraheem () and his angel visitors started some verses before) as that of the angels it is fair to ask, why did they attribute the decree to themselves if they are obedient and not the source of the decree or command?
Imam ar-Razi has an answer for that in his a Tafssir al-Kabir:
In the following I will translate from Arabic to English language as these translations are mainly of my own take them with the necessary care!
المسألة الثالثة : لقائل أن يقول : لم أسند الملائكة فعل التقدير إلى أنفسهم مع أنه لله تعالى ، ولم لم يقولوا : قدر الله تعالى ؟
The third issue: Someone could say: Why did the angels assign the act of determination to themselves, even though it belongs to Allah Almighty, and why did they not say: Allah Almighty decreed?
والجواب : إنما ذكروا هذه العبارة لما لهم من القرب والاختصاص بالله تعالى كما يقول خاصة الملك : دبرنا كذا ، وأمرنا بكذا والمدبر والآمر هو الملك لا هم ، وإنما يريدون بذكر هذا الكلام إظهار ما لهم من الاختصاص بذلك الملك ، فكذا ههنا والله أعلم .
The answer is: They only mentioned this phrase because of their closeness and their elitism to Allah Almighty, as a king's dignitary would say: We planned this, and commanded us with this, and the mastermind is the king, not them, but they want by mentioning these words to show what they have of closeness with that king, so it is in our case here, and Allah knows best.
A description of the story
To sum up: The angels came to visit Ibraheem() informing him about a couple of things among which was their mission to save the people (believers) who followed Lot ().
Why were they sent?
because Allah Has decided to punish the disbelievers and save the believers by bringing them out of the city or area which will receive the punishment!
Since Allah knew that Lot's wife was among the disbelievers the angels also informed Ibraheem that Allah already has decided and decreed (since Allah knew her behavior we can safely speak of a determination or predestination), that even though she may be part of the saved party she will receive the punishment as she won't follow the orders given to the people who should be saved.
Since nobody else, but Allah and His angels knew about the upcoming punishment before the angels were sent in a mission to first inform Ibraheem () then save the believers the angels in their speech to Ibraheem () used the term "We decreed" or alike instead of saying "Allah decreed to punish and we will follow his orders to save the believers before the hour of punishment" for either of the two reasons:
- "We" is used as a honorific as they speak of Allah with high esteem and respect.
- They used "We" es explained by Fakhr a-Deen ar-Razi to express their closeness to Allah, yes Allah decided, but they were the first to know and since they will follow some orders to fulfill the punishment (saving people who shouldn't be involved since they are believers) they used "We" to express that they are fulfilling orders of their Lord of whom they speak respectfully!