5E handbook specifically states that warlocks can have antagonistic relationships with their patrons. I see no reason why a Neutral Good character, pressed into desperate circumstances and with no other options, can’t make a deal with a devil, selling their soul and damning themselves in exchange for power and knowledge. Also, a warlock doesn’t need to summon their patron. Rule book says the patron can approach them first.
There are no alignment restrictions in 5E, specifically to encourage role play. Want to be an Evil paladin or a Lawful barbarian? Have at it. So a warlock who is a reluctant agent for a fiend, chaffing under the moral compromises they have to make in this service, is totally a legit character.
Last edited by Warlocke; 12/07/20 08:30 AM.