This is not really an answer or an explanation, but yours is more of a rethorical point than a question. It is rather pointing out that these kinds of explications are needed for many different texts, a person into religion may have already read some.
A quick google search for gruesome passages of the old testament gives, among (many) other things:
Samuel 15:2,3: This is what the LORD Almighty says: 'I will punish the Amalekites for what they did to Israel when they waylaid them as they came up from Egypt. Now go, attack the Amalekites and totally destroy everything that belongs to them. Do not spare them; put to death men and women, children and infants, cattle and sheep, camels and donkeys.'
Hosea 13:16: The people of Samaria must bear their guilt, because they have rebelled against their God. They will fall by the sword; their little ones will be dashed to the ground, their pregnant women ripped open.
This would surely be very offensive to the Amalekites or to the people of Samaria, if some were alive today.
The "explanation" for this hadith is likely to be very similar than the ones you will find for jewish texts. It may include:
"You are not concerned": this verse only applies to the end of times/3000 years ago, or if a tree starts speaking, ...
"You are protected by verse XXX": There are some verses you could pick that require someone doing a crime before being killed
"This does not apply to all jews, by verse YYY": pick a verse about jews converting themselves.
If you ever read some apologetics for judeism, transpose them to this hadith instead of asking, you will save some time.
PS: I usually don't like "whataboutism", it is not the goal of this answer. It is rather pointing out that he may already be familiar with the kind of "explanation" required.