All of the humans, titans, and animals had glowing red hearts. I belive that that represents "the source of all organic lifr" that Ymir had found to become and create titans
I don't think it represents the titans or titan shifters, there are too many holes in that theory in my opinion. I think it's about cruelty, like the example Mikasa gave when she realized how cruel the world is when she watched the Praying Mantis eat the Butterfly, its no different when the dinosaurs are eating each other, and how titans eat humans.