The drawing does invite concern, and it would be prudent to arrange an evaluation with a pediatric mental health clinician. What did your son have to say about the picture? What prompted the anger toward the other child? You might also have a talk with the teacher and see what the teacher's perspective is on your son's relationships with peers.
I definately wouldn't ignore it! Oh boy, I would get him into see a coucelor, but make sure it's someone that works with children, behavior therapy, or anger management issues. You might also want him to see a psychiatrist to see if he may have some sort of disorder (not trying to be rude here) such as bipolar, or a behavior disorder, etc. That's definately a red flag! I would talk to my ped dr. and ask for a refferral and don't let them dismiss this, tell them you are concerned and want to seek help no matter what they think. Not all peds Dr's are informed in mental health issues.