AFAIC, it can't. This has been discussed before and, as frankthedm posits, some people feel you can use an evil (or "just feral") power source for good ends. I don't see it that way. If you really wanted to be good, every other class (not just paladin) can be "Good" with a capital-G. You could have been a Wizard or a Ranger, but you chose to make a deal with a demon.
As others have mentioned, you could write into your character history that you were "tricked" into entering into the pact, or somehow "destiny" caused you to be a Warlock, or your parents sold your soul to a demon on your behalf (the plot of a current TV show, apparently), but really, how often can this happen? As character backgrounds go, I think it would get old to always have to come up for reasons why you're not evil.
And that still doesn't explain why you keep using your ability to ruin people's souls. Not evil? I don't think so.
Yup.