This is the 17642nd most frequent Russian word.
убитые
"Uбитые" means "killed" or "dead" in English.
In this sentence, 'убитых' is the past passive participle of the verb 'убить', meaning 'killed'. It is used as an adjective to describe 'множество' (many people).
Война оставила множество убитых за собой.
The war left many killed behind.
Here, 'убитых' again acts as a past passive participle modifying 'солдатах' (soldiers), referring to soldiers who were killed.
Этот фильм о убитых солдатах, защищавших свою страну.
This movie is about the killed soldiers who defended their country.
In this case, 'убитым' serves as a predicate adjective in instrumental case linked to 'высмотрел' (looked), metaphorically indicating being overwhelmed or shattered by grief.