Think: Loki. Coyote. Raven. Set. Anansi. Eris. They aren't stupid, they aren't suicidal. They seek to increase the chaos in all situations, and to exemplify it in their own actions. They will tear apart the powerful, they will prop up the weak, they will rip apart strategies and defy expectations. They do not want things to be easy or seamless, they want messy, gritty unpredictable.
A slaad appears in town -- maybe it tripped through the wrong gate. What does it do? Well, seeing all those nice ordered buildings and nice, orderly streets...it wrecks them. These structures, these constructs, these icons of artificial order, it turns them into jumbles of debris and non-functional shells. The people inside? Well, it invites them to help! And if they don't help...maybe it eats them. Or ignores them. Or starts out doing one and ends up doing the other.
Or maybe it sees a crowd of people in a boisterous marketplace. Soothed, it walks into the marketplace, eats the fruit vendor, and starts selling fruit in his place. Sometimes it charges 1 copper piece for half of it. Later, it charges three souls and a cookie for half of one melon. At the end of the day, it jams the leftover fruit down the throats of anyone who is left in the market and writes the words "FREE FROOT" in their entrails.
The slaad is a menace to society. It cannot abide the cooperation and organization and construction. It is a creature of alien chaos, a source of horror for those worlds that have the misfortune to be touched by them.
What do they want? To sow discord, to wreck order. What motivates them? A need for chaos, a comfort with change, and a visceral loathing of pattern and authority. What do they act like? Like malevolent whimsy, like gleefully violent sidhe, like a sociopath in a clown suit. They have their own desires, and they seek to fulfill them, and to encourage others to fulfill their own.