Matthew Poole and continuators on 1 Kings 11:25

25 And he was an adversary to Israel all the days of Solomon , beside the mischief that Hadad did : and he abhorred Israel, and reigned over Syria .

He was a secret Enemy, watching all occasions to do them mischief cunningly and privately all that time; and when Solomon had forsaken God, and was forsaken by God, he shewed himself more openly and maliciously.

So the sense is, This infelicity was added to the former concerning Hadad, mentioned above v. 14, &c. Whilst Hadad molested him in the South, Rezon threatned him in the North. But some understand this of Hadadezer, who is here called Hadad, by way of abreviation, (which is not unusual in proper Hebrew names, as is well known) and that for, or because of (for the Hebrew Particle eth, is sometimes put for el, which oft signifies for, or because of, as Hebricians know) the evil which befel Hadad, or Hadadezer, i. e. he bore a grudge against the Israelites from, and ever since the slaughter that Ioab made in Hadadezer's Army, whereof he was a member, although he also took that occasion of making a defection from his Master.

Over all that part of Syria, enlarging his Empire more and more, and thereby laying a foundation for much misery to Solomon's House and Kingdom.