TT #50 · Parashas Noach · Fourth Aliyah · Passuk 9:5
But for your own life-blood I will require a reckoning, from every beast I will require it, and from man; from every man, his brother, I will require a reckoning for the life of man.
Torah Temimah notes the Yerushalmi reading that a Ben Noach may be executed on his own admission.
“And of man” — this teaches that a Ben Noach is executed even on the basis of his own admission.
Custom (self-incrimination accepted in Noachide law).
Unlike Israelite courts, a Ben Noach’s own confession can suffice for capital liability (per this reading).