(ROS777;3269)
I don't believe he gave in to a grumbler
Then it's hard to understand why grumbling was brought up at all. Scripture does not say that Cain grumbled, and if one is looking for a reason for God changing His mind, having compassion (which of course He can, and does), one might well (and charitably) say that Cain was genuinely afraid for his life (which of course he did not deserve, but God is merciful). It may have been that God did not concede at all, but merely explained.
What I was wondering about, which started this topic with my first post, was why was not the death penalty imposed, as in the Old Testament when one murdered another with forethought.
That wasn't the problem presented; the issue was whether murder was considered sinful. God does not kill murderers as soon as they commit murder, any more than He kills thieves, adulterers and false witnesses as soon as they offend (though He has done sometimes). There was no reason to pick on Cain, and there was no more mercy shown to Cain that there is to us when we hate another. Indeed, Cain is our model in some respects.The Israelites were commanded to exercise the death penalty for various offences, but there were two reasons for that, as I see it. One was to ensure purity of standards within Israel, as Israel was supposed to be a witness to God's laws to surrounding nations (which they were, sometimes). The other was to show that sin is extremely serious, and that God punishes for sin. Jesus was to make this explicit when warning that sin is much more important that what may happen to the body, but it must have occurred to both Israelite and gentile that the conscience is eternal, and that the God of the Hebrews would hold men to account.
As to your previous point
Answer the post concerned directly, please.