He's become obsessed with the proto-molecule to the point of jeopardizing the crew and obligation to help the refugees. This season was a turning point and has exposed some of the more interesting skeletons in their closets.
Oh yeah, something that's great about this series so far is that almost every single character has a unique and realistic backstory, and they are all have their flaws. It's beautiful the layers to these characters.
I personally don't find an issue with Holden's PM obsession, as he rightly sees it as the greatest risk to their way of life mankind has ever encountered, and it is entirely unpredictable and apparently capable of way too much crazy. He is rightly fixated on doing everything he can to eradicate it, and I suspect if he ever learns of the other sample still in existence after it was decidedly not hurled toward the Sun, there is going to be some big fallout. As he has been so close to it and personally witnessed its potential, I think he has every right to feel the need, as a captain of a gunship, to do what he can to protect humanity out of, hell, instinct really. He's proving he's up to the task and not cowardly, and he now has a mission, and knows what he, his crew, and ship can do.
Yeah you have to step away to look at things from a new angle, but at the same time, others can do that as well and offer input to the captain. A good captain is expecting that, and to be made aware of issues that he may have missed so that his decisions take everything into account.