Are we ever 100% sure though, when the possibility exists that the prosecution is corrupt and the confession could be coming from someone that is mentally ill?

Are we ever 100% sure though, when the possibility exists that the prosecution is corrupt and the confession could be coming from someone that is mentally ill? And stepping aside from morality, if it costs 10X[0] the cost of a life prison sentence to kill someone, isn't it really better to just put them away for life with no chance of parole? [0] 10X is a made up #, but it's certainly a large multiple of non-capital punishment cases.