Well there are plenty of ballads that deal with infanticide, so I don't doubt that it was common, especially in light of the fact that there was no effective birth control and an unmarried woman who gave birth faced social ostracism, disgrace and could very well find it difficult if not impossible to support a child on her own. But, I've never heard that attitudes towards the practice were lax.   

Regarding Mary Hamilton - I wasn't really referring to conflicting feelings about going to the gallows - everybody would view that prospect with dread, but her feelings about the whole situation - guilt for what she had done along with anger at having been used and then cast off.   I think there are several very rich scenes in the ballad - the verse about the provost's wife and the bailiff's wife who express sympathy, and she tells them 'you need not weep for me, for had I not slain my own wee babe this death I would not die", the scene with the king described above, the scene where as she faces the gallows...What can I say, I've always liked the song!