My son just turned 5... we found a couple baby birds in a nest. the next day he took a rock and killed them all.. is this behavior to worry about?

See his doctor: Some of this answer depends on his response to his actions. Was he sad about what he did? Did he understand what he was doing? It would be a good idea to talk to his doctor who hopefully knows him well - you and your doctor can determine if this was a curious behavior or indicative of a more problematic underlying issue.

Aggressive behavior: Talk to your son and let him know what he has done. Reprimand him but explain why he is being punished. By the same token, reward him for good deeds.Try channeling his aggressive motor activity into outdoor sports or hobbies