Tori or nage is responsible for the safety of uke. To me, there is no excuse for deliberately injuring someone. Once the person has tapped out, it is a deliberate attempt to inflict injury to continue applying the technique beyond that point. It may be ok to stop the application of more force and hold the pin at the tap out point for control purposes but to continue applying more force risks permanent or serious injury.
Such aikibullies exist. You see them at seminars and they usually injure someone. Frequently, nothing is done about it and that's unfortunate. They lack control. Yet, do it back to them and they cry foul.
I have had one permanent wrist injury from an instructor's sankyo. There was no excuse for the injury other than his ego that day.
Accidents happen but part of zanshin is awareness of the capability of your partner to take ukemi and the joint's ability to take strong locks.