As a rule when the T-killer cells are in action, the pathogens are viruses which implant their RNA in a cell to infect this one. With the implanted RNA new viruses can be created inside the cell in order to break out (lysis) when there is a great number of them. They in turn are able to infect another cells in the same way. To prevent this apoptosis is the real deal in that case before it gets worse. Perforin and Cytotoxin are released to initiate this process - all these steps are doing before enough viruses are created inside the diseased cell.
In apoptosis the diseased cell is shrinking, the organelles (as in the ribosomes, the mitochondria, golgi apparatus, etc.) are eungulfed by vesicles and the macrophages eating these remains of the cell with all the produced viruses in.