Was driving west on Katella, when I saw a tanker truck with its left blinker on. I slowed down because I assumed he wanted to merge into my lane. The driver behind me began to honk in an aggressive manner. He then got in front of me and began to slam on his brakes while giving me the finger. This man is mentally unstable and potentially dangerous. He is a white male in his mid to late 50s with balding grey hair.