I am sure that the slaughter of Egyptian children was perhaps the most merciful thing for them too. Somehow, Chirst is not in that picture!
Perhaps it was, we don't know.
I have lost a child so I perhaps have a different outlook on God's will.
Psalms 137:8 O daughter of Babylon, who art to be destroyed; happy shall he be, that rewardeth thee as thou hast served us.
9 Happy shall he be, that taketh and dasheth thy little ones against the stones.
I don't quite understand that verse, but I accept that God can do no evil, and I accept the fact that I don't always understand why God does something.
1 Chronicles 16:34 O give thanks unto the LORD; for he is good; for his mercy endureth for ever.
BTW, in the Authorized Version, commonaly called the KJV, the phrase "his mercy endureth for ever" is found 41 times in 41 verses.