Gavilar is an Alethi, and their culture is war. He united the princedoms the only way he knew how - by conquering them.
I agree that Gavilar didn't start on his conquest with the best of intentions, but I think for him it was culturally appropriate: all the princes strive for dominance over each other, and we see this throughout the books with how much difficulty Dalinar has keeping the other highprinces in line, with how they constantly engage in border skirmishes at home even though they're technically united as one kingdom. I don't think Gavilar could have united them by anything else other than force, and even Dalinar comes to take a similar view later on.
We also know that Radiants, despite their vows are not morally perfect. In Dalinar's flashback, the man he believes is Nohadon states: "Alakavish was a Surgebinder. He should have known better. And yet, the Nahel bond gave him no more wisdom than a regular man. Alas, not all spren are as discerning as honorspren."
Perhaps Gavilar believed that a united Alethkar (even if by war) would fare better against a Desolation than separate, squabbling princedoms. We also know that Gavilar behaved oddly by Alethkar standards, especially near the end, that he believed in the Codes on a personal level even if he did not impose it upon his kingdom. Perhaps that was enough to start him down on the path to become a Bondsmith, even though he probably never got as far as Dalinar did.
I agree that Gavilar and the Sons of Honor do appear like scumbags and everything we see them do has been pretty questionable, but I don't think we know enough to cast judgment upon them at this point.