You're treating it as though Levi was telling him that it was the right decision. I didn't see it that way.
To me, it seemed like he was saying "Transforming isn't wrong, neither is trusting us. There is no right or wrong, there's just the decisions you make."
Like I said, I have no problem with him trusting them the first time. But when that trust he placed in them turned into a complete clusterfuck failure, he should never have let them talk him into trusting him a second time.
I blame Pleadingeyes McTreepancake, for getting him to do it twice.
Remember that Mikasa IS a noob as far as they're concerned. She just joined, like, last week. She's a very skilled cadet, but still just a cadet.