So in my head Dustin sees Angela being horrible to Lilith, tries to help, nothing does and so he gets close to Lilith to help, ends it with Angela.

Lilith would go to Dirk's to escape her home WHILE Dustin would go to Angela's to escape his. So Dustin saw how horrible they treated Lilith while Dirk ONLY heard about it so he doesn't really know how bad it actually is.