After Scarlett told Jacob that she was pregnant, he did what he thought was right, by telling Katrina about it. He did not love her or anything, but he thought she was on the need to know basis.
But, leave it to Katrina to f*ck things up. Had Jacob known that she would have gone to the club to see Scarlett, he would have kept the news to himself. He didn't want any more drama between Scarlett and him, and if it turned out to be his child, which oddly, he thought it would be, he would love for them to have a peaceful relationship for the sake of their child.
Jacob was kind of pissed when Scarlett texted him, telling him to warn Katrina not to put her hands on her. Katrina had no right to do that, and Jacob had to let her know. She can't do as she pleases whenever she pleases.
Jacob left
