We talk over the clues: their weapons, maps, and essential gear are still here: their packs are full, so they almost definitely weren't looted...unless the person or people that did the killing left their former equipment in their victims' hands. There aren't any zombies around.
We piece together a vision of the horror that went down: roof man must have been alone inside the tower and heard the other two survivors coming. With nowhere to run but up, he took the high ground. When they reached the top, the roof survivor got nervous and got the jump on them. The pair of survivors returned fire, but died where they stood. The roof survivor didn't die immediately—he must have slinked up the the roof to bandage himself, but by then he'd lost too much blood, collapsed, and died.
I reflect a little on how special this is: we've created a story in our minds about what happened here based on some evidence. Then I remember what I'd do if I were gunned down in a wild west-style gunfight in a zombie-filled Czech castle: I'd respawn and marathon-run it here—or send any nearby friends—to recover my gear. We need to get the hell out, now.