You mean the same Cole banter where even he thinks he will never have their blood washed off his hands?
There's more to it than that, and maybe you could do your due diligence and go back in the thread and find what was posted when we engaged in more serious debate/discussion about this topic. I really don't need to write personalized essays for every single person who can't be bothered to go through the thread.
You're ignoring additional dialogue from Cole which makes it fairly clear that Rainier was unaware that the children would be there - that he was effectively duped into it himself (just like he duped his own men), and that by the time he found out, it was essentially too late to stop things from happening. Per Cole's line about the axes hammering at the carriage door and all that.
Yeah, he participated in a massacre - he admits it, everyone knows it, and we know it. But maybe you missed the running theme of second chances and redemption in the game. And should you keep Blackwall around, a lot of the companions work through it themselves and come to an understanding that he truly is a changed man.
I myself called him a self-interested pr--- for taking that money and for what he did AND for the person that he used to be, but he's not that same person, and he's done enough to show that he deserves a chance to atone.