  We study the impact of merging supermassive black holes (SMBHs) on the
central regions of massive early-type galaxies (ETGs) using a series of merger
simulations with varying mass ratios. The ETG models include realistic stellar
and dark matter components and are evolved with the GADGET based regularized
tree code KETJU. We show that observed key properties of the nuclear stellar
populations of massive ETGs, namely flat stellar density distributions (cores),
tangentially biased velocity distributions and kinematically decoupled
(counter-)rotation can naturally result from a single process $-$ the scouring
by SMBHs. Major mergers with mass ratios of $q>1/3$ produce flat, tangentially
biased cores with kinematically distinct components. These features originate
from spin reversals during the decay of the SMBH orbits caused by gravitational
torques from tidally expelled nuclear material. Minor mergers ($q\lesssim1/3$)
on the other hand, form non-rotating cores and the tidal reversal becomes less
important. Low-density stellar cores scoured in (multiple) minor mergers are
less tangentially biased. This implies that the nuclear stellar properties of
massive ETGs can be solely explained by stellar dynamical processes during
their final assembly without any need for `feedback' from accreting black
holes. We predict a strong correlation between decoupled cores, central
anisotropy and merger history: decoupled cores form in binary mergers and we
predict them to occur in elliptical galaxies with the strongest central
anisotropy. Measurements of the central orbital structure are the key to
understanding the number of mergers a given galaxy has experienced.
