as salaamu alaykum,
I can understand that it may seem harsh, but this ruling is the exception rather than the rule. If Allah (swt) wanted, He could have made it part of the Shari'ah that ALL prisoners of war, at any time and in any circumstances, should be immediately killed. But we know that's not the case - in most cases, prisoners of war were given many options and are also dealt with in a very decent and humane way, and ransomed even at times at a very paltry sum - in some incidents in the seerah some were freed for simply teaching some sahaba how to read and write. There are many examples of this throughout the seerah and the life of the khulafa ar-rashidun. The question is, what makes this incident different? Why such a "harsh" ruling when our deen is in it's normative state one of mercy and ease? There must be something here, wouldn't you say?
It's just like in the Quran - we find that every surah begins with Bismillahir Rahman ar-Raheem (In the Name of Allah, The Merciful, the Compassionate). Every single surah begins this way - except one, the 9th surah, Surah Tawba. Knowing that, we have to come to the conclusion that there is something particular or special about that surah that makes it different, not that we can draw a conclusion about the whole from this particular part.
I think it's a matter of understanding the historical context of the situation. The prisoners of war that were taken were people who were openly and actively aggressive and hostile against the Muslims; and who would simply return after being freed to that same type of aggression, at a time when the Muslims were still in a vulnerable and precarious situation. In one commentary I read they describes these POW as "arch-criminals" who had tortured and killed innocent Muslim for their faith.
You have inspired me to re-read Martin Ling's book on Seerah, which is one of my favorites for its writing style and for somehow capturing the emotion and passion of the events that took place. I would definitely recommend reading it, it gives a different take than most other books which I've found sort of dry, over-detailed, etc. There's also a great tape set by Sh. Hamza Yusuf of Muhammad
btw, an awesome duaa that was often said by the Prophet
is the following: Ya Muqallib al-quloob, thabbit qalbee 'alaa deenik
. O Turner of Hearts, keep my heart firm and steadfast on Your deen.
It's such a beautiful duaa, that shows that our hearts can change so quickly... even the word for heart in Arabic, qalb, implies something that moves, flips, changes. So we have to constantly ask Allah to keep our hearts in a state of firmness and steadiness, immersed in recognizing Him, loving Him, remembering Him.
May Allah grant all of us steadiness and sureness on this path,