In this exposition of what kares means, the following view is quoted:
[...] R. Yosef Kafach (Mishneh Torah, Hilkhos Teshuvah ch. 8 n. 4) offers a different interpretation. He suggests that the Rambam’s view is that kares is an eternal suffering of the soul.
Several questions regarding that:
Eternal suffering means it lasts an infinite amount of time. A billion, trillion to the trillionth power, 10^number of atoms in the universe...any finite number is as actual 0 compared to infinity. So according to this view, this would be a completely disproportionate punishment compared to all the other views (e.g., dying young or even losing share in olam haba or even destruction of the soul). Unlike any of those, this punishment and suffering would just never ever stop. Could that really be what was meant and is it compatible with Judaism's conception of Divine mercy (even for the most heinous crimes)?
This punishment is much greater than just being executed by the court. Execution by the court is a relatively quick demise and grants atonement, and enables the person to retain his olam haba. Kares on the other hand, potentially (according to some views) makes one lose a share in olam haba or have his soul be destroyed entirely, or (according to this view) subjects the soul to eternal suffering.
How can that be, since kares is generally thought of as a lesser punishment than execution by the court. For example it appears lower in the order of Rambam's eight levels of punishment (see here and other answers there too). Also, often kares is given for the very same sins as execution by the court, except when a technicality (like witnesses) was missing. Surely the absence of witnesses shouldn't make the crime more heinous, and to the degree that the punishment would be so much more severe than just execution?
So what's going on here? Is kares a lower [or even basically equal] form of punishment than execution by the court, or is it, counter-intuitively, a much higher one (perhaps infinitely so)?