If i was in Sam's shoes I would first just try and learn more about what the black panthers are. If I knew they were a violent group i would decide if i thought there was another way. But, if his whole life he had been watching his dad protest with out violence and nothing change I can understand that he must feel discouraged. so maybe in that discouraged state it would be a lot more tempting to protest with violence, and even more so when whoever they are being violent against started it. For example when Bucky got beat up I do think that would be an appropriate time to use violence. It gets to a point where its just self defence.