He states it because if one cared to misrepresent Christianity or Judaism with long ignored texts like some do Islam one could have similar results.
And dont forget that does not change the moral quality of the persons who did do those things way back then.
If Samuel were alive today would he still slaughter Agag and the royal family, including women and children, of the Amalekites or not?
Does that make Samuel evil?
Gods truth does not change and to attack someone for what the prophets themselves did is to set oneself against God, no?
Samuel's "slaughter" might be more figurative than literal today, such as a hostile takeover, or exile. There have been cases of entire families killed - the Romanovs (Tsar) in Russia, by the Soviets - but we do not slaughter defeated enemies anymore.
Slaughtering entire Royal/Ruling families was accepted then because it was "normal procedure" then, it reduced the need to keep watching one's back, at least from that direction.
The point of the matter is that Judaism grew out of the bloodthirsty stage;
Christianity did for a while , but has had relapses at times, generally against fellow Christians (Protestant v. Catholic, Orthodox v. Roman, etc).
Islam has always had the Draconian punishments in the Islamic lands ruled by Sharia law: floggings, stonings, beheadings and amputations, and the like. That is the difference.
"Gods truth does not change and to attack someone for what the prophets themselves did is to set oneself against God, no?
No, it is not, because there is no moral equivalency between the two.
The Biblical occurrences were thousands of years ago, and are NO more. The Islamist occurrences of punishment under Sharia law are TODAY, and that, the cruel and unusual punishments as described above, is NOT acceptable in civilized society.