I think killing sleeping monks is an evil act in this case. I believe this was murder for personal gain. The PCs were promised 'the opportunity to gain "great powers over air and sky".' It appears the PCs were not carrying out the mission for their commitment to the rival order, but to increase their own power.
What I think is not overly relevant as I am not the GM in this case. Where I am the GM I allow the players to play their character and if there are alignment repercussions I let that fall as I see fit. If, despite alignment, the paladin was behaving according to code, repercussions would be minimal. If the paladin was not behaving according to code I would be designing the next part of the campaign to include an atonement quest where the paladin could explore the meaning of the code fully.
I also take the view that one evil act does not automatically result in a change of alignment. A pattern of behaviour does. I would look at this act in the context of the adventure and take into account the previous conduct of the PCs.
In the Pathfinder campaign where I play a paladin, my character would not even have taken the mission. Infiltrate and assassinate are contrary to his code, and he would have suggested other approaches first.
The final aspect to this is the game rules. The original post does not say what system is being played. Some rules have a very low tolerance of what constitutes an evil act. The 3rd edition Book of Vile Darkness included lying (sometimes), cheating, theft and greed in the list of evil acts. Therefore if the GM in this case thinks that killing the sleeping monks is an evil act, I wouldn't argue.
