First: The "medic" is meant ironic. He doesn't fling syringes either. He throws knives and looking at his weapon and general gameplay, it seems he is a poison mixer. Which would explain why the "medic" title was always in exclamation marks, unlike the other titles. Being a poison mixer would actually fit quite well to a traitor.
Second:
https://i.pinimg.com/originals/ff/c3/47/ffc347dfb1ac24d67901944b14b2923f.jpg
These are some concept arts for the splashart for Omega Squad Twitch. In panel H, we see the hands of what is most likely supposed to be one of his victims. Do you see how these hands just have 4 fingers? A while back, all Yordle were unified by a splash art update to have 4 fingers. So this is most likely a Yordle he is killing here.
It is not a clear evidence that he is a traitor, since we don't know the enemy that the Omega Squad is fighting. But that it seems like he is killing a Yordle in that splash art is highly suspicious for me.