I still find it difficult to believe this was deliberate aerobatics, considering the experience and ages of the pilots
Experienced folk often have an exaggerated sense of their capabilities. A roll can be made with little "g" generated, but a loop is something that by its very nature requires a "g" load that can easily get out of hand if hamfisted. The aircraft in question was only rated to 4g, an aerobatic aircraft requires a 6g rating. The report is very telling, you don't perform rolls, especially loops, in such a low powered aircraft other than by intent.
After the aircraft returned from its earlier flight it was scheduled to do another flight – the accident flight. Prior to the accident flight, the Instructor was heard to remark to the Pilot “I suppose I better take the loose bits out of it”. The pilot who had flown the aircraft earlier asked the Pilot about his intended flight, and he responded “I’m going up to do spins”. Later, in the same conversation, there was some general discussion about aerobatics, including a query about the maximum “g” that would be experienced during a barrel roll, which the Instructor opined he would expect not to be more than 2g..........
The Investigation contacted a number of witnesses and obtained details of their observations of the aircraft, from a variety of vantage points, and at various stages of the accident flight. One witness observed the aircraft some minutes prior to the accident and noted that the aircraft executed a number of consecutive roll manoeuvres. In addition, several witnesses reported hearing waxing and waning engine sounds, as the aircraft manoeuvred. Other witnesses reported that immediately prior to the accident the aircraft executed three loop manoeuvres. During the descent from the apogee of the third loop, two loud noises (variously described as bangs or cracking sounds) were heard, parts were seen to separate from the aircraft, and the aircraft began spiralling downwards.