Solomon was hardly the saintly kind of person. Him having multiple wives was probably only a small fraction of his unseemly activities.
(1) There's a good chance that Solomon actually usurped the throne from Adonijah. First, Adonijah was older than Solomon, and would have been naturally in line before Solomon to be king.
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2 Sam
3:3 His second son was Kileab, born to Abigail the widow of Nabal the Carmelite. His third son was Absalom, the son of Maacah daughter of King Talmai of Geshur.
3:4 His fourth son was Adonijah, the son of Haggith. His fifth son was Shephatiah, the son of Abitail.
3:5 His sixth son was Ithream, born to David’s wife Eglah. These sonswere all born to David in Hebron.
5:14 These are the names of children born to him in Jerusalem: Shammua, Shobab, Nathan, Solomon,
5:15 Ibhar, Elishua, Nepheg, Japhia,
5:16 Elishama, Eliada, and Eliphelet.
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We know Absolom was killed. Amnon was dead (2 Sam 13). This made Kileab next-in-line then Adonijah, though we don't know what happened to Kileab.
In any case, Adonijah doesn't seem to know anything about Solomon being the rightful successor to David (1 Kn 1:5-6). Not only did Adonijah seem to think he was the rightful king, but David's own top general/right-hand-man Joab and high priest Abiathar also thought Adonijah was the rightful heir (1 Kn 1:7). While David was on his death bead, Adonijah held a coronation party just outside of Jerusalem with David's other sons (except Solomon), the elders of Judah, and David's own royal court in attendance. Everyone thought Adonijah was the rightful heir - everyone except Solomon, his mother Bathsheba, Nathan, and some of David's elite warriors. And it's suspiciously Nathan, Benaiah (one of those elite warriors), and Bathsheba that go see David while he's on his death bead. And while they are in there, it's only then that David supposedly names Solomon as his successor. This is suspicious indeed. Bathsheba has ample motive to see her son be the successor to the throne given that David had her husband murdered (also one of David's elite warriors). David's elite warriors also have motive - Solomon was the son of one of their own, unlike Adonijah.
After Bathsheba, Nathan, and Benaiah claim that David named Solomon, they rush Solomon into a coronation ceremony and then rush him back into Jerusalem before Adonijah could arrive. They have to do this to try to raise support quickly since the support for the rightful king seems to be in favor of Adonijah.
For whatever reason, Adonijah seems to have panicked and grabbed the horns of the altar in the temple, which should have given him safety according to Israelite law. But Solomon and Bathsheba betray that law and murder Adonijah. Then they murder David's top general/right-hand-man Joab. Then they dismiss David's high priest Abiathar. All are conveniently replaced by Solomon's supporters. Benaiah becomes top general. Zadok becomes high priest.
All appearances are that Solomon usurped the throne and eliminated the original claimants. It looks like a classic coup. This isn't all that surprising either; this frequently happens when "strong-man" types of rulers, like David, die leaving a power vacuum behind. Factions formed between those who aligned with Solomon (Nathan, Bathsheba, some of David's royal guard) and Adonijah (David's general, David's high priest, David's other sons, and the royal court). In all odds, David's royal guard did what they always had done, murder their way into power and installed one of their own.
(2) Solomon also seems to have enslaved some of his own population in order to build his work projects - the temple, his own palace, walls, and various cities (1 Kn 9:10-24). Of course, comfortingly we are told that he didn't enslave Israelites, just non-Israelites, though later we find out that he also actually enslaved some Ephraimites and Josephites (1 Kn 11). So in reality, Solomon just didn't enslave his own Judahites. Solomon wasn't wise enough to prevent one of the leaders of his slave labor (Jeroboam) to revolt. Not only did Jeroboam lead a revolt, but it was successful in the end - Israel separated from Judah.
In any case, Solomon appears to have been a usurper to the throne, a murderer, an adulterer, an idolater, a lawbreaker, a politician, a pawn (for his mother and David's royal guard in the beginning), and a harsh slave master.
But other than that, I'm sure he was a model example for the rest of us.