I totally understand that when you're out and about and you've finally got your baby to sleep that you inwardly think "please shut up" when someone else's LO is crying/screaming. But glaring and tutting and rocking your buggy more and more aggressively to make clear your baby is sleeping and you are irritated is not really ok. Unless perhaps the other LO's parent is ignoring the screaming while they smoke crack. You'll have noticed I wasn't doing that but was quite clearly trying to soothe and placate my poor almost 14 week-old who is teething and, like many babies, struggling with the hot weather. Your baby was very new so I'll give you the benefit of the doubt that you've yet to experience that moment when your baby is having a total meltdown in public and nothing you can do seems to soothe him/her. I promise you it will happen and you will be incredibly grateful for anyone who's nice about it and sympathetic. I hope you don't come across anyone like you who made a stressful situation ten times worse. Believe me, I wanted her to stop screaming more than you did.
I cried on the way home so well done you.