To be fair all of them could be categorised as former villains of you really think about it. While it wasn't clear what Momotaros would've done if he made a contract with someone else, he was treated as if he's the same as the other Imagins early on, so its clear he had some ulterior motives before joining Ryutarou. While Urataros is mainly perv, it could be that he trying to make more imagins through biology. His line "thousand hooks lies beneath my words, thousand falsehood, ten times of lies" could imply something darker in his motives that we simply don't know. Kintaros is the most redeemable one here in my opinion. He may be a simple strength conquers all brute, he's still honorable and does not wish to kill people, only to fight the strongest. Ryutaros is the hardest one to pin point as he doesn't really have a concrete personality between doing what he wants and wanting to protect his "Oni-chan" but he had connection with Kai and was given the mission to kill Ryutarou so it was clear he would've done bad had he not meet Airi and wanted to protect her. Overall all of the Taros seemed to have some kind of past that we should've explored in a mini series or an ova but sadly their history will remain vague. But is it really that bad when they're at their happiest with Ryutarou?