Read the fluff for Infernal warlocks. It says that a race of devils created the pacts, but Asmodeus destroyed them and wiped their names from mortal memory. "But you study their perilous secrets anyways."
So really you're just tapping into the deals, the packets of power, that those devils forged long ago.
You know how you steal someone's cable or internet? Just tapping into their cablebox/plugging into an outlet, running a wire to yours, and tada, you're leeching off their system? Same deal, except with arcane power.
A "Good" warlock might believe that there's a finite source of this power, and by using it for "Good", it's being used up so some evil warlock or devil can't take advantage of it. Or it could be seen as putting Hell's resources to good work, tieing up some of their power that they can't use to do something bad.