The end justifies the means
And "you can't make an omlet without breaking a few eggs". In simplest terms, it is the law of the jungle. Might makes right. Except in this case, we are talking about the morality of your particular cause (might) and since your cause is moral and just, whatever you do to further that cause is just (right).
The basic premise has been with us since the first proto-human ripped up a plant or killed an animal because he was hungry. And since Og bashed Thag and took the food away from him, because he was hungry.
When we became "civilized" we created rules which govern the morality of our actions. Whether it is the rule of law, or the will of the gods, these rules state which actions are considered just, and which are unjust. It is the general agreement of the people to follow these rules that creates and allows society to function.
There is a strong tendency for mankind to consider those who break the rules as evil. Most of the time that is how we define evil. This carrys over to lesser things as well. We tend to think of those who disagree with our positions (rules) as wrong (because, mostly, they are), and it is a small, indeed, a very small step from there to bad, and from bad to evil. The greater the moral importance of the cause to you, the more likely those opposing it are considered evil.
And since they are evil (and no doubt in league with the Devil - by whatever name you know him), then whatever harms them must be "good". Someone keying your car because of your bumper sticker is at one end of the spectrum, and terrorist bombings the other.
The only solution that has proven to work is codifying the forbidden acts (laws), and punishment for breaking them. AND punishment only works as a deterrent IF the potential perpatrator believes the punishment WILL happen to them, AND it is severe enough to deter them from committing the action contemplated. Most criminals do what they do because they do not believe they will be caught. And even if caught, the punishment is not more than they can easily bear.
There is very little deterrence from "slap on the wrist" justice. Fines and even jail time are not deterrence to some people. For them, the only solution is to remove them from society.
All else being equal (and it almost never is) bigger bullets tend to work better.