Killing is acceptable when the alternative is worse.
If I know someone is going to be engaging in rape and abuse for a time into the future, and the damage they cause will be significant, preventing that damage is a priority. If there are better solutions than killing, they should be employed instead - if there are not, killing is the most acceptable course of action - assuming it has no additional negative side effects, of course, that would make it worse than simply "killing".
If someone is going to suffer for weeks before perishing, and wants their miser to end now, it is acceptable to kill them, and for the same reason - it is better than the alternative.
Considerations such as whether the person is "guilty", or whether they "deserve" to be killed, seem to me as nothing but mere cruelty, and a sign that the killing is likely to be not only wrong but entirely unacceptable. Killing someone for your own emotional gratification or enjoyment is morally repulsive at a fundamental level, and this is what most calls for executing rapists or murderers boil down to. When dealing with matters of life and death, such things as guilt are trivialities - death zeroes them out. The person no longer exists, and the only bits that persists are the event that result or were avoided. We will all die, every one of us, after all - death is no great evil, but it is a tool that evil can wield to achieve it's ends, and the opportunity cost of killing is immense.
In the same vein, collateral damage is unfortunate and regrettable, but sometimes required when the alternative is worse. Engaging in activities where you know some of those you are responsible for will die is essentially killing them, but is also sometimes required for the same reason. It is wrong (unless they themselves take responsibility for the result - bringing about your own death in the hopes for benefits is not really wrong), but it may also be acceptable if the situation demands it.