The Doomguard (nicknamed Sinkers) was one of the "official" philosophical and political factions of Sigil. Their belief was that the decay and entropy of everything was not only inevitable but a good thing, and what was "supposed" to happen.
Though they did not generally act only to destroy everything they came across, they were in favour of the ideas of entropy and chaos and may well have been disposed to help them along in ways they saw as suitable. They were also not likely to worry much about the cessation, decay or destruction of anything, since that would have been against their tenets.
They found common ground with the Bleak Cabal, a faction that believed nothing had any meaning, and the Dustmen, who believed this life is nothing but a bleak afterlife to be transcended by reaching oblivion. On the other hand, the Doomguard was at odds with the Harmonium and the Fraternity of Order, two factions that placed much importance on order.