See Rashi, Bereishit 34:25 D"H "Sh'nei V'nei Yaakov"
That "They failed to act like sons [of Yaakov] in that they didn't seek advice from him [namely Yaakov]"
This represents an interesting parallel to Nadav and Avihu - two "righteous" brothers - who failed to consult their elders - namely their father Aharon and their Uncle Moshe before bringing an "eish zarah" into the Sanctuary.
While Shimon and Levi were only castigated and were not executed, the parallel is nevertheless worth mentioning.
Recklessness COULD have been prevented by a simple consultation with the elders that they SHOULD have consulted in the first place.
Thus, in addition to the improper behaviour, one must also factor a form of insubordination or usurpation of authority into the equation