Just to take away any lingering doubts you're having, there are four people involved in the train robbery/child murder.
Jesse is beside himself and has to get out of the business because of the child murder. Walt is trying to use what happened to emotionally manipulate Jesse, though unsuccessfully. Todd is a deranged sociopath who committed the actual crime.
Then there's Mike, whose only concern is how inconvenient it would be to retaliate against the guy who just murdered the child.
Every single time Mike doesn't kill someone, it's because he's either been ordered not to, he's concerned about what they might know, or because he's been offered more money not to do it.