It sounds like they had already had a very challenging festival, and one by most accounts they handled remarkably well (especially when compared against events like Y Not etc), but as you say this could be the straw that breaks the camel's back.
In all fairness, I'm not really sure how Bestival could've prevented this? If somebody has that inclination, they'll do it, unless they find there were substances involved that specifically contributed to what happened. Still absolutely appalling to hear, RIP to the individual's family and friends. Nobody should go to a festival and not come back.