Sirius Black told Harry that his younger brother Regulus “panicked” and tried to back out of being a Death Eater and was killed because Voldemort demanded “a lifetime of service or death” (OP6). He also thought his brother’s life was thrown away and “not very important.” The real story was quite different, and Sirius underestimated his brother.
Regulus was shocked when Lord Voldemort left Kreacher the House-elf in the sea cave to die. Through special House-elf magic, Kreacher survived, but Regulus was a changed man. He figured out that Voldemort’s plan to go “beyond death” involved the use of a Horcrux and that’s what was hidden in the sea cave (DH10). He vowed to go back there and destroy the object or die trying.