Catelyn is one of my favourite characters. She is a mother through and through and goes above and beyond for her children, not only out of duty but out of genuine love. Everyone is getting bent out of shape over her treatment of Jon Snow. Catelyn saw him not only as a reminder to Ned’s (alleged) infidelity, but also as a threat to her trueborn children’s futures. Ned acknowledges Jon Snow as his son, legitimizing him, which means Jon could later on legitimately make a claim on Winterfell and all that was to be promised to her children. Catelyn hates Jon Snow not because of who he is, but of what he represents: infidelity, dishonor, and a legitimized threat.
Cersei Lannister is no different in this respect and goes a step farther in actually having Robert’s bastards killed (even one that was only a baby). Yet most people here on tumblr seem to idolize her, when in fact she is the crueler and more ruthless of the two.