> I was under the impression viruses became less lethal because if they became more, they'd kill off their hosts and be less likely to spread.

Only if the lethality is negatively impacting their fitness. If it can spread to another host quickly, who cares if the previous host dies. There's no selective pressure against it.

Which is why Delta is scary. Not just because of the higher viral load, but new strains could be more lethal and still not be selected against.