Only with the Dark One's Dagger (Once Upon A Time) can the Dark One be killed, the name of the killer appearing on the dagger's blade, granting them the powers and title the Curse of the Dark One onto the user.
Reinhard Tristan Eugen Heydrich (Dies Irae) can use the abilities of everyone in his Legion as well as those that are killed by his Legion or his lance, Longinus Testament.
Upon defeating the Robot Masters at the end of each level, Mega Man (Mega Man series) is able to obtain their signature attacks for himself.
Immortals (Highlander) can absorb the knowledge and power of any Immortal that they defeat and behead in single combat, a phenomenon known as the Quickening.
Cinder Fall (RWBY) absorbed the power of the Fall Maiden by killing Amber, the previous Fall Maiden.
Marceline (Adventure Time) stated the only power her demon heritage granted her was the ability to suck souls, like her father. This allowed her to gain the specific abilities of vampires she had killed.
Steel Inquisitors (Mistborn) as hemalurgists, gain their powers through killing a Metalborn person with a spike and implanting that spike in their own bodies, taking on a weakened form of their abilities that depends on how long it has been outside of blood or a body.