There are SEEMING inconsistencies in the Bible but all have been dealt with. The above, both are true, the sin of the father does extend to the children, for he teaches them, alcoholism is an example. However, this chain can be broken, the son decides to drink in moderation or quit altogether, and teaches his son abstinence or temperance. It only teaches that we are each responsible for our own choices, our own sins, but that if we sin, our children will very likely follow our example, if we curse, are abusive, arrogant, liars, adulterers, blasphemers, theifs, etc. our children may be, too.
The God of the Bible is Love. The people He destroyed were wicked beyond imagination. If the firstborn of Egypt were slain, they were simply slain, not cast to Hell. God is love and loved us enough to pay for our sins Himself on the cross. As there are Physical Laws, there are also Spiritual Laws, such as the penalty for sin is death. Sin breeds death, that is just a spiritual fact. So, God in His great love for us stepped up and died for us while we were yet sinners and enemies of a Holy God. He did this and offers eternal life freely, to completely, absolutely undeserving sinners such as we are, a free gift from a loving God.