To complete missions given to them by the government
The heads of S.H.I.E.L.D. (Nick Fury, Maria Hill, Tony Stark, Phil Coulson, and Clay Quatermain) got together to discuss the growing problem of criminals. Phil pitched the idea of forming their own team of former criminals to take out the bad guys. Four out of five heads voted this a good idea. They hunted down Deadpool, Elektra, Punisher, Venom, and Red Hulk and forced the five of them to join the Thunderbolts Initiative.