So according to evolutionary theory we are not comparing whether or not a DNA sequence provides better reproduction than an ideal theoritical code but only comparing the codes that happened to appear by random chance
So if among a subspecies of a species a code that happened to cause more harm to outsiders with no benefit never competed against a code that did not cause harm to outsiders unless there was a benefit there would be no method to select for the code that harms the host less
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Diseases that kill their host faster have less opportunity to reproduce
If leeches instantly killed their host instead of letting their host live will extracting blood they would be called a predator instead of a parasite and leeches would be less succesful at survival to reproduce more leeches if they did not get more blood killing per time by the host then by letting the host live
@shortstories
It was explained in the parable of the scorpion and the frog. It's their nature: parasites always act as parasites.