‘And they said, “Should he deal with our sister as with a common prostitute?” '
They are justifiably indignant. It is they who have been wronged. What else could they do and retain their honour? Their sister had been treated like a common prostitute, available for men whenever they desired. The tribe had been insulted and violated. The covenant had been besmirched. We may decry what they did, but men in their day would have fully understood its necessity.
We may pause to consider that sometimes the way of compromise is necessary, but when deep sin is involved such compromise is unacceptable. In the terms of their day Simeon and Levi were justified in what they did. And by it, although it was not their motive, they protected the ongoing of the covenant and preserved the purity of the tribe. As ever God moves in mysterious ways in the bringing about of His purposes.
(It would in fact be a mistake to assume that the patriarchs never killed anyone. It was sadly a normal part of life when people were wealthy and vulnerable. We certainly know that Abraham would have done so in rescuing Lot and all the patriarchs had riches and herds to defend and we can be sure that attacks on them were many. Their men were trained to fight for that very reason. Bloody fights would have been fairly commonplace. The difference here is that a whole town (but very small by our standards) was involved. But as we have seen the circumstances were very special.