Why do politicians engage in negative campaigning?
Because negative campaigning works.
Why does negative campaigning work?
Because negative campaigning encourages people to hate.
Why do people hate?
Early humans evolved the capacity to hate other humans. Hate must have helped the haters to survive and reproduce. How?
Hate motivates people to fight.
An early human who fights may be able to take territory, tools, stored food, mates, etc from other early humans. An early human who fights may be better able to defend territory, tools, stored food, mates, etc. Thus an early human who fights may be more likely to survive and reproduce.
On the other hand, an early human who fights might die in a fight and not reproduce. An early human who fights will be less likely to cooperate with other early humans, and will be less likely to receive the benefits of cooperation with other early humans.
So sometimes early humans were more likely to survive and reproduce if they fought, and other times early humans were more likely to survive and reproduce if they cooperated instead of fighting.
How did early humans know when to fight and when to cooperate? This requires intelligence. The early humans with greater intelligence would have been better at deciding when to fight and when to cooperate. Thus the early humans with greater intelligence would have been more likely to survive and reproduce.