This is the 2430th most frequent Arabic word.
قتيل
"قتيل" means "killed" or "victim of killing."
Here, 'قتيلًا' refers to an individual who has been killed, describing their state as the object of the sentence.
وجدوا قتيلًا في الحقل.
They found a slain person in the field.
In this usage, 'القتيل' identifies the topic of the prayers, emphasizing the person's state post-mortem as the subject of the noun phrase.
يدعون للقتيل بالرحمة.
They pray for mercy upon the slain.
'قتيل' is used here in a possessive manner ('قتيل المعركة'), indicating association with the event (the battle) that led to their state.