Abstract The recent discovery of high-redshift ( z > 6) supermassive black holes (SMBH) favors the formation of massive seed BHs in protogalaxies. One possible scenario is the formation of massive stars via runaway stellar collisions in a dense cluster, leaving behind massive BHs without significant mass loss. We study the pulsational instability of massive stars with the zero-age main-sequence (ZAMS) mass and metallicity , and discuss whether or not pulsation-driven mass loss prevents massive BH formation. In the MS phase, the pulsational instability excited by the ϵ -mechanism grows in . As the stellar mass and metallicity increase, the mass-loss rate increases to . In the red supergiant (RSG) phase, the instability is excited by the κ -mechanism operating in the hydrogen ionization zone and grows more rapidly in . The RSG mass-loss rate is almost independent of metallicity and distributes in the range of . Conducting stellar structure calculations including feedback due to pulsation-driven winds, we find that the stellar models of can leave behind remnant BHs more massive than . We conclude that massive merger products can seed monster SMBHs observed at z  > 6.