But actually, in canon, every Slytherin Voldemort supporter we see turns against Voldie when s/he realizes Voldie is a direct threat to a loved one:
Snape/Lily
Narcissa/Draco
even, to an extent, Lucius/Draco- the last battle of H.
Draco/his parents(?) - trying to avoid identifying Harry to the Snatchers, trying to keep Crabbe from killing or hurting Harry in the Room of Requirement

So the only objection to Regulus/Kreacher being a part of the pattern is that you don't credit Regulus with caring that much about the house elf who may well have been his nanny and/or pre-Hogwarts primary playmate....