We can explain that when David repetitiously said, 'Hashem, how numerous are my tormentors! Many rise up against me', he was referring to the double misery that his son brought upon him; the spiritual agony of almost transgressing the terrible sin of idolatry, and the physical pain and anguish of a son rebelling against his father.
The question thus arises; why didn't David say 'how numerous are my tormentors' concerning the wars of Gog and Magog, since that too will cause the Jews to sin?
The answer is that, because the 'travails of Mashiach' will be so trying and arduous upon the Jews and will leave them with no peace of mind, it will be considered as if they were coerced to sin and they will not be fully responsible for their sinful behavior. Hence, their sins aren't considered as their 'torments'. On the other hand, David's travails will not give him that leeway, for although Avshalom's rebellion indeed left him with no peace of mind, nevertheless, because David himself was behind his son's discipline, it was as if he himself was to blame for his son's wayward and rebellious behavior; thus he cannot use his son's rebellion as justification for his sins.
Accordingly, we can explain that when David indicated that the war of Gog and Magog will be less painful than a degenerate child, he was referring to the spiritual pain. David was suggesting that the spiritual pain which Gog and Magog will bring upon the Jewish people, by causing them to 'cast off from themselves their cords and yolk of Torah', won't be so painful because the Jews will not be fully responsible for those sinful behaviors. On the other hand, the spiritual pain brought about by a degenerate child is so much more painful, for the father is fully responsible for any sinful behavior which is brought about by his rebellious son.
Thus, Rashi explains as follows. The reason why David did not say 'Hashem, how numerous are my tormentors!' in regard to the wars of Gog and Magog, and thus indicated that in the spiritual sense it will be less painful than that of a rebellious son, is because of that which David said, 'Why do people gather, and the nations talk in vain?', where he alluded that the Jewish People won't be fully responsible for their misdeeds which Gog and Magog will bring upon them.
