I think it's about someone contemplating suicide, probably the guy who wrote it, and when he's high as a kite, on drugs obviously, he may have the confidence to kill himself, and when he's low (withdrawing - this suggests the drug is Heroin) it's so bad that he thinks this may be the right time to kill himself, to end the pain and misery of withdrawing from Heroin. He is suicidal whether he's high, low, or normal.
I think it's about someone contemplating suicide, probably the guy who wrote it, and when he's high as a kite, on drugs obviously, he may have the confidence to kill himself, and when he's low (withdrawing - this suggests the drug is Heroin) it's so bad that he thinks this may be the right time to kill himself, to end the pain and misery of withdrawing from Heroin. He is suicidal whether he's high, low, or normal.