English Translation
You are killing me slowly.
"죽이고 있어" can mean "I'm killing [someone/something]". In this case, the verb "죽이다" means "to kill", and the "-고 있어" ending indicates that the action is currently ongoing.
"죽이고 있어" can also mean "It's killing [me/you/him/her/them]". In this case, the verb "죽이다" is used figuratively to mean "to be very painful or exhausting", and the "-고 있어" ending indicates that the feeling is currently ongoing.