Seems that the mistreatment involved was lack of cleaning the animal cages. Taub, going away, had hired two caretakers supposed to do this in his absence. He had also hired Pacheco, which is, as you say, how Pacheco got access.
Bottom line, the two caretakers didn't do their job. Pacheco failed in not alerting Taub to what was going on. He did alert authorities.
Not exactly perfect, but a far cry from getting inside and abusing the animals. Even not alerting Taub might have had a perfectly valid explanation - you ever tried running down someone off at a conference or something?
Lack of care sounds simple but it’s not. In this case it’s extreme abuse. The animals involved had had surgery involving the nervous system with the intention of showing that the brain could remap itself. (The redone experiment did in fact show that with important implications for the rehabilitation of brain injuries and brain disease.)