Obviously using the term "criminal" tends to automatically infer that there was some level of intent involved, but that isn't necessarily true. The term "criminal" can be broad and could technically be applied depending upon what the investigation uncovers. Possibly under the broad definition of "criminal neglect" by the pilots actions. I'm not saying that is the case or that conclusion will be reached. The investigation may just determine it was an unfortunate accident and likely pilot error, but not at the level of criminality.
From the views I have seen, it appears the smaller plane came in along the wrong path, the pilot tried to correct by accelerating to gain lift to avoid the collision, but it was too late.
I wonder how much flying time he had in this plane. I assume it was quite a bit. He was definitely an experienced pilot in general. The guys that fly these planes love what they are doing and try to get as much time doing it as they can. It is an amazing hobby and lifestyle. Certainly an unfortunate event.