If people were 100% rational, would the world be better?

[Angrboda]

If we're homeostats... living creatures with a tendency towards a stable equilibrium - and cognition is a physiological response - then the bare minimum normative case would suggest that a living, competent, and uncompromised cognitive agent should/would register a strong selective preference for life.

Which is to say that the choice (and subsequent mountain of natural behaviors) follows from premises genuinely apprehended by that agent, premises which we would grant without qualification - whereas ideation to the contrary would strongly suggest that something has gone horribly awry for that agent.

Consider this, while we might easily accept any number of rational cases for life, of the relatively few rational cases for death we might accept - do we imagine the person to be thinking clearly?

If we go down the route of framing it in terms of what is physiologically normative then we enter the realm in which our choices are simply biologically determined and choice is no longer an issue. In some sense, one has to grant the fiction of free will for the discussion to have any meaning. Once you do that, there's no rational reason to presume the biological tendencies as any kind of norm.