How would you handle a jealous violent toddler bullying my infant son?

The bully's mom was holding my 5 month old. I didn't know her son was a bully at the time. My 5 month old was quiet while being held and of course drooling on himself. Her 2 and a half year old was viciously and aggressively stabbing a piece of paper with a pen and he had a mean look on his face. When his mom wasn't looking he kept with that vicious look on his face like he was jealous of his mom holding my son, he quickly tried to violently stab my son with the sharp pen. It made a mark on my son's arm and my son's skin turned red and then my son started crying. The mom did not take the pen out of her son's hand and just told him to say "sorry" and let him continue stabbing the piece of paper with the same pen. My blood was boiling so I told her I wanted to hold my son and she insisted on holding him and said everything would be okay at which point I insisted on her giving me my son and said we had to leave. My infant cried for the rest of the night. He acts different now. He saw a rowdy, loud aggressive kid in an elevator that looked similar to that bully and my son acted aggressively and gave him a very angry look and pounded his fist on his stroller and in that kids direction like he wanted to hurt him. I turned my stroller the other way because the kid looked aggressive like he was going to run over and do something. My son also has been acting out by pulling on the bumper pads in his crib and kicking them which he never used to do before this.He used to be happy go lucky and friendly to all. I've never seen him act that way toward a toddler or in his crib until that stupid incident happened with that violent toddler and his stupid ignorant mom. I'm hoping my son will get over this, hopefully forget about it and be more happy go lucky again. My blood is still boiling over this as I am a new mom and didn't know other kids could be so violent and parents so neglectful of their kids. I'm venting more than anything but also wondering what others would do in this situation or if situations like this have damaged their kids. I'm hoping my son isn't ruined from one bully.