Appeared to take good care of his son when people were watching. Who knows how many times, when he was in charge of Cooper, he was so busy sexting, he 'forgot' he had Cooper, only in the past, it didn't result in injury or death?
And I know you know this, but people do kill people they love. Fathers sometimes take out their entire family...out of a twisted definition of love. And people let people die they purport to love. We can't actually know what Ross's emotional capabilities are.
We do know he can purport to love Lena, and screw around with multiple people behind her back. (and yes, I know people do this to people they love, but there are those who could NEVER do this to people they love. He ain't in that category.) He can put on a 'good father' hat all he likes, but I'm guessing the father of the 15 year old he was sexting would argue a good father wouldn't sext with minors, or want their minors sexted with. A good father would make that connection. I'd argue a 'good father' wouldn't risk hurting Cooper with the prospect of jail time for sexting with a teenager. Or consorting with prostitutes.