You miss one very important point here. The father and mother did not have the final say. The elders of the city did. The mother and father were to sieze the disobedient son and bring him to trial, but they could not order the execution. If the child were convicted by the city elders, then the execution would be carried out by the state, not the parents. See Deuteronomy 21:18-21 (And besides that, Jewish tradition holds that this law was never enforced.)
I really don't have the time to go over all the reasons why putting a disobedient child to death might be a good idea. I will suggest a couple: 1) Any child that refuses to obey his own mother and father is not likely to obey God or any other authority. 2) Any person who has the unmitigated gall to actually try to harm (strike, curse) his mother or father is very likely a threat to the rest of the community as well.
I'm not going to call God's law evil or ridiculous, nor am I going to make excuses for the people who chose to disobey it, for
"Woe to those who call evil good and good evil;
Who substitute darkness for light and light for darkness;
Who substitute bitter for sweet and sweet for bitter!
Woe to those who are wise in their own eyes,
And clever in their own sight." Isaiah 5:20,21
> Woe to those who call evil good and good evil;
Words that someone who suggests murdering a disobedient child *really* needs to pay attention to.