DHC6 Pilot reported observing a MU-2 purposely overflying a C-150 that had just landed; resulting in a NMAC.

Narrative

I was in Aircraft X as PF with an IP (Flight Instructor) as PNF on a training flight. We were holding short of RWY XX. Multiple aircraft in the pattern; as much as 5 aircraft. Most were Cessna and Piper training aircraft. Very busy airspace. Aircraft Y reports into the frequency; telling another aircraft that he 'will be right on top of them' and asking their intentions. I assume many bewildered since from this point on; non-standard communications were used and plain language was utilized to communicate. Aircraft Y wanted to land amidst all of the other; much slower aircraft; complains that ZZZ Approach put him there. Eventually joins the pattern. A Cessna 150 was approaching the runway; and lands. Meanwhile; Aircraft Y conducts his own approach right behind the C150. He could have known on the base to final turn that he was too close. He approaches and flies over the runway; comes to what looks like within 100 feet (vertically) of the C150 and goes around. My IP tells him over CTAF that that was not necessary and was reckless. Aircraft Y responds and says that he was just doing an approach and went around. Aircraft Y rejoins the pattern. He turns downwind to base; cutting off what looked like 2 aircraft that were on base and base to final turn; respectively. Makes another low approach. Eventually; space clears up in the pattern; and my aircraft departs from RWY XX with no issue.