Not "him", but "it"
"It" most likely refers to "the house of Jeroboam", for destroying Jeroboam's house, which he did in 1 Kings 15:25-29.
Consider the ASV and the footnote [a]
Also, consider other translations...
...in being like the house of Jeroboam, and because he struck it.
...also with footnote [a].
...in being like the house of Jeroboam, and also because he destroyed it.
I know Greek, not Hebrew, but I do know that pronouns in Greek and Hebrew can be a little more flexibly applied. So, this is a very reasonable vagueness in Hebrew. That is okay and expected in those languages because they expected to look back to whatever the "him/it" would have stood for and thereby know if it should be "him" or "it".
but if not "it"...
Even if it was meant to be "him", it would then point to Jeroboam by the association with the house he just killed. And, that brings everything down to the final part of the final sentence—the main point in this part of the passage...
The exegetical idea to take from the end of v7 is: though he killed Jeroboam and his household, he was still just like Jeroboam and his household, making him all the more detestable.
As a note about choosing a translation, generally, technical word studies like this will work better in English with the NASB. Dr. Ken Taylor once told me, "If we want to study the words, we need the words."