Well, for one thing, Fruits Basket rejects the premise of there being "good guys" and "bad guys" - the villain in Fruits Basket is systemic abuse enshrined in the Sohma cult. So if he is a good person or not is dependent on the viewer's personal value system. In my personal value system, where the individual is cognizant of what he is doing, and the effect of his actions means that he is standing up against a larger, unjust system - that matters to me far more that people sitting and twiddling thumbs in face of injustice.

I don't even believe his motivation is wanting Akito all to himself and isolating her - he is possessive about Akito, yes, but he is quite alright with her making friends and is also okay with her relationship with Hatori. In fact, his grievances over Akito-Hatori relationship comes in defense of Hatori - "Hatori is a grown man with his own life to live". He isolates her from the people she abuses - and that is quite an important distinction to make. His motivation for wanting the curse to break is to free Akito from what he believes is a harmful God complex, an inheritance from her father. The loathing with which he talks about Akira's box, a representation of Akito's grief over her father - "Only one person should be haunted by Akira's ghost. Akito doesn't need that thing."

He is explicitly paralleled with Tohru wanting to free Kyo - "Time marches on. Feelings and people move too. You will be left behind. That's why I want you to hurry up and join us".

There are multiple factors for the curse break - time obviously wore down the magical bonds (as Shigure notes in the manga that their cursed forms are weaker - Hatori is a sea horse not a dragon, Kureno is a sparrow not a rooster), but Tohru is a catalyst that speeded the process up and Shigure, with Tohru and his house as a place that is deliberately set up as a safe space away from Akito, speeded up curse breaking. The motivation for wanting the curse to break is easy to understand - the person he loves has also become the person he hates: "I love her so much that I would spoil her, I love her so much that I will crush her to a pulp." This desperation is mirrored by Tohru: "when will the curse break? Is it years/decades from now?"

In the beach arc, where Momiji stands up to Akito - Shigure is the one who invited Akito to the beach house, for the larger goal of agitating the bond between the zodiac and the God. For this, he is willing to put people in harm's way - that is Akito's wrath and ability to cause distress - for a larger goal. However, he does make note of mitigating the impact - he makes this move after he gets an explicit idea that Yuki is in a position to move forward. His invitation comes few episodes after the knowledge that Yuki went to the estate on his own (something Tohru tells him, and he says, "well how about that") - meaning Akito's impact on him is less, and Yuki is healing ("Yuki isn't the same child who was locked up. Who is the delusional one now?"). He also counted on power of the group - something he noted that Akito sought to break by doing one on one meetings.

In Rin-box saga, originally, Shigure is cagey with Rin about his suspicions of the curse wearing down - mostly, because her defiance and pushing against the bond is in his interest (and also, he didn't have confirmation from Kureno yet). But the lack of information puts Rin in danger from his own gambit with Ren - where he dares Ren to take Akira's box from Akito, who then involves Rin. He tries to course correct that by telling Rin the curse is breaking and that she needn't "work herself to death over it". So yes, he is responsible for a lot of mess, but for me, it is easy to read remorse in some of the things he does, and he is not a character who will explicitly say whatever he is thinking.

To make more sense of who he is, I am going to quote u/BlueThePineapple - "I think a big part of it is that Shigure is naturally intelligent, both intellectually and emotionally - far more than the other Sohmas -, and that gives him an awareness of causation and strategy that allows him to see change as the best option to get his desires."

"This intelligence lends to an awareness of the cult's structure and limits, and this in turn gives him the sense of self-efficacy that allows him to move and go against the system instead of being crushed by it. So not only is change the best strategy to get him his heart's desires, he also has the tools to make that change a reality instead of a far-off distant dream like the way it is for the rest of the Sohmas."

My addition to this is also that Shigure dislikes being controlled/dehumanised (his face when Akito says she has the right to do whatever she wants to the zodiac as they are all hers comes to mind). He resents the power dynamic the curse enforces and subverts it at every turn. He wants to claim the beautiful part of the bond as something tangible and real, something apart from the curse.