The first devils were celestials created to fight the Devils. But along the way, they got corrupted and started to collect any souls they could for the war. Asmodeus made his deal with the other gods in order to secure this. As a result, devils seek to corrupt others and turn them away from their gods. This gives them more fodder in the Blood War.
Devils adhere to a very strict hierarchy and will always obey those above them to the letter. They also seek to replace those of higher status by outmanoeuvring them. They are expert manipulators and will abuse every rule they can get away with without breaking it. They are also completely ruthless towards those of lower rank. Both because they can, and because they know their underlings are conspiring against them.
The Nine Hells aren't a pleasant place. This makes devils even more spiteful against everyone else. And as long as you keep getting more souls of lower rank, you yourself is better off. Exactly what you have to contribute depends on which Archdevil you serve.