A person has “control” (ba’alus) over his property, he can make his property hefker or gift it or destroy it, so because he has control over his finances, therefore he can place upon himself financial obligations (except for the financial obligation of a “knas” [fines] which is only in authority of Bais Din to impose). His hodaah can therefore be michayiv him financial obligations.
In contrast, a person is not a baal habos over his body, he cannot commit suicide, injure himself or even permanently deface his body (tattoo) , therefore he has no baalus to obligate himself capital punishments or physical pain (malkos). [This is besides the fact that such punishments are in the exclusive domain of Bais Din – as pointed out re: knas].
The question I always had, does Torah COMPLETELY rejects the idea of “confessions”, that a confessor to a murder is disbelieved (because “Ain Adom Maysim Atzmo Rosha”)?! Granted he is not punished based on his confession (which requires a criteria of aidus and has’roah), but is he not believed that he is indeed the murderer (is his testimony totally rejected and his confession is rendered meaningless)?!
I won’t compare it to viduy, when a person confesses his sins, but shouldn’t his confession to Bais Din (a viduy of sorts) be accepted as part of his teshuva – that he is accepting responsibility for his past wrongs?!