Pathogens will evolve to best survive and replicate - for fitness. If a pathogen kills a patient quickly before much chance of spread then yes they wouldn't be

Pathogens will evolve to best survive and replicate - for fitness. If a pathogen kills a patient quickly before much chance of spread then yes they wouldn't be very fit and mutations delaying death would be beneficial. However this certainly does not imply that all pathogens evolve to become 'mild'. Killing the host (or permanently incapacitating them) is not necessarily incompatible with effective spread. Indeed, we have no strong evidence of a pathogen evolving to become milder. We do have lots of examples of hard won population immunity reducing the harmfulness of pathogens. Unfortunately it has become a bit of a 'natural is best' trope that viruses necessarily become weaker, but in fact nature will quite happily slaughter us, and it is our artificial interventions that keep us fit and healthy.