Why cannot man be his own judge (St. Thomas Aquinas)?

Question

St. Thomas Aquinas, Summa Theologica II-II q. 64 a. 5 ad 2:

One who exercises public authority may lawfully put to death an evil-doer, since he can pass judgment on him. But no man is judge of himself. Wherefore it is not lawful for one who exercises public authority to put himself to death for any sin whatever: although he may lawfully commit himself to the judgment of others.

I know that Aristotle (who influenced Aquinas a lot) said in Politics book III, part IX, that

Most people are bad judges of their own case.

But are there other reasons why Aquinas was against the idea of ​​a person being his own judge? Did he comment on this somewhere else?

Answer 1

Because man is prone to (at least) two things: self-delusion and survival.

Self-delusion may manifest in a person underestimating the severity of their crimes, or in failing to acknowledge they have acted maliciously at all.

Such self-deception may in turn be a manifestation of the survival instinct, against which the self-destructive impulse (such as the recognition of culpablity) rarely survives.

A person in power is almost certain to be corrupted by such impulses when they become the subject of allegation, or even before anyone else becomes aware of their misdeeds.

So whilst in theory the virtuous man or woman might be capable of condemning themselves publicly, it is unlikely if such condemnation goes against their interest and/or the interests of their offspring or other family or friends.

Of course, not all judgement is of a criminal sort, but we constantly delude ourselves in all aspects of life frequently via a need for cognitive ease and because of the Illusory Truth Effect.

This makes us unreliable and/or unfair judges of our own behaviour, which is why a Magistrate will never sit in judgement of his own decisions.

EDIT: To reflect @Criglcragl's comment, self-preservation is sometimes overcome by an impulse to unnecessary/irrational/unfair self destruction and/or self castigation. Wherever our emotions lead us, any sense of objectivity to which we aspire is inevitably polluted by our cognitive circumstance, a circumstance into which we only have limited (and often fleeting) insight.