Oh Richard wrong again. Evolution is the change in allele frequency in any given population. It has no value judgement. It is random chance that a given allele imparts an advantage in a given environment. There is no evolutionary advantage whatsoever to being a heroin addict. Darwinism and evolution say nothing about co-operation. There may be an advantage to being heterozygous for this deleterious trait, which is usually the case, and is why the allele persists in the population.  The death of these individuals would not even reduce the allele frequency in the population. It has been argued that it would actually increase the allele frequency, because without the burden of parents caring for the homozygous deleterious individuals with manifestations of disease, the population of heterozygous individuals increases. AKA if you abort all hemophiliac individuals, but the parents have the same number of children, the frequency of the allele increases not decreases.