He might be, it is not likely that he was just killed off screen. The manga has not shyd away from showing front and center how the killer monkey killed the humans so it would be very weird that the author decide to hide how the killer monkey killed or incapacitated Hasegawa. Unless its a ploy by the author to plant suspitions on Hasegawa while he really is dead.
Personally, I think there are two conspiracies at hand. The first and most obvious one is the murder conspiracy created by the killer monky and his would be allies. The second one is the company sabotage conspiracy. If you remember this whole company retreat started because the medicine company was facing a crissi after a drug scandal, so I believe that the source is that a small group of the company's employess were bribed by a rival company to sabotage a product. And one of said group might be using the monkey to kill his co-conspirators to tie up loose ends. Wheater he hired the killer or is just taking adavantage of the situation is too soon to tell