Was it morally right (then) to kill someone for cursing their parents?
Is it your intention to suggest that God is unjust?
First of all, it's not a matter of simply killing people. It's a public, government sanctioned, execution of a convicted capital criminal.
And yes, of course it was morally right to execute those whom God said should not remain alive.
Ezekiel 13:19 And will you profane Me among My people for handfuls of barley and for pieces of bread, killing people who should not die, and keeping people alive who should not live,....
And finally, as I have already explained multiple times (I think to you), this law did not apply to small children or smart mouthed teenagers. This law had to do with adults who were drunkards and went around defaming their parents publicly. Further, there are good arguments to suggest that this law was symbolic and had to do with Israel's relationship with THE Father. This law was paralleled when Israel rejected the risen Messiah. In response God "cut them off" because of their rebellion (Romans 9). I've heard arguments on both sides though and while I do fall on the side that says that this law would not have just application outside of Israel's covenant with God, I wouldn't be dogmatic about it either way when discussing these things theoretically.
Clete