In Scooby Doo (2002), Scrappy-Doo wants Scooby-Doo's protoplasmic soul to complete final ritual take over the world.
And the leader needs to absorb a purely good soul to complete the ritual.
Why Scooby-Doo's protoplasmic soul considered as pure?
Humans are nasty greedy violent creatures. You saw how Shaggy got jealous about the girl, even when his lifelong friend tried to help him. Scooby, on the other hand, has the traits of an innocent loyal dog. He would never betray his friends (on purpose, at least) and he loves them all. He is not violent, he doesn't lie, he would never let down someone in need. He doesn't like being the center of attention, he doesn't boast, and despite being a big coward, he finds the courage to do the right thing.
He is quite the opposite of Scrappy, actually. Scrappy is an annoying inconsiderate jerk, who wants to take over the world.