I have no problem with Matt becoming a villain. And yes, he was too good to be true. But there was no context to what happened. I mean, usually, there will be clues that you can point to when a character changes that much. Like with Olivia ending up with Andrew Swanson. That works because we know they were friends and that he had great affection for her even though he wanted Anna. Look how genuinely pleased he was to see her with Baby George in the hospital.
Maybe it was supposed to be a case of he came literally out of nowhere, had an impact on their lives, and then disappeared as quickly as he appeared. It's also ironic that he told James that his son and the child's mother "severed all ties" and disappeared and then he ended up doing the same thing.