Four short lines carry a painful realization: the person who tried to heal someone ended up being harmed by them. The speaker sets up a neat reversal. "I was your cure, but you were my disease." The two roles mirror each other, and the mirror is what stings. One person offered care while the other, apparently, offered damage.
The speaker seems exhausted and hurt, but also a little bewildered. The second pair of lines, "I was saving you, but you were killing me," uses the past tense, which suggests the relationship is over or at least understood differently now. The title, "Killing Me!", with its exclamation mark, adds a raw, almost shouted edge, though it can also sound like a sigh of frustration.
The mood is bitter and blunt. There's no decoration here, only a plain statement of an unfair trade: the more the speaker gave, the more was taken. The poem leaves us with the sense that trying to rescue someone can cost you yourself.