Empathy

See this article for more information about empathy itself.

Empathy is the capacity to understand or feel what another person is experiencing. It is a normal emotion that is wired into the minds of pretty much every living thing...

...except Bloons.

Bloons
For things like monkeys, apes, and humans, empathy is an emotion wired in their brains to ensure they don't do things that destroy their race, as these animals depend on working together in social structures to survive. For Bloons, however, this emotion is normally nonexistent. Empathy exists as a rare "disorder" in Bloons, and only 1 in 8,750 inflations will generate a Bloon with such "disease". Bloons with empathy often show "symptoms" such as hesitating or refusing to kill monkeys, even simply looking sad when seeing someone in sorrow. "Normal" Bloons actively try to isolate and capture these "empaths" and kill them, as they see it as a threat to their existential purpose.