Or HaChaim on Genesis 49:5/אור החיים על בראשית מ״ט:ה׳

1 שמעון ולוי אחים, Shimon and Levi are brothers, etc. Jacob stressed the fact that Shimon and Levi were brothers as their similarity was natural, they shared many attitudes, something that is not always the case between brothers. It is also possible that Jacob referred to their joint action in Shechem as well as their plan to dispose of Joseph at the time, when he described their brotherliness. Jacob may have meant that these two between them shared the element of fire in an exaggerated manner. Our sages in Yuma 22 describe that Saul committed one sin which cost him the kingdom whereas David committed two serious sins without forfeiting his kingdom. The reason for this was that Saul was even-tempered by nature whereas David was impetuous by nature as we know from Samuel I 16,12 where the prophet describes David as אדמוני, red cheeked, i.e. hot-blooded. When G'd judges people He makes allowances for their natural tendencies, their genetic mix. Although someone who contains a greater than average share of the element of fire will be most enthusiastic in his service of G'd as a result (as we know from the tribe of Levi), this enthusiasm does not act as a barrier against his committing errors on occasion. Such errors also reflect his genetic composition. The fact that a person such as Saul was of an even temperament usually acted as a barrier against his falling victim to the lures of the evil urge. When Jacob described Shimon and Levi as אחים the word is derived from חם, heat. Jacob cursed their anger because when it erupted it was apt to prove unusually destructive. Anger is rooted in the element fire.

2Once such anger is kindled it results in excess heat and that is why Jacob compared their anger to כלי חמס מכרותיהם, the use of weapons to commit violence. By stressing the word מכרותיהם, (from the root הכר, something one is conscious of), Jacob indicated that Shimon and Levi's tendencies to commit acts of violence were not something they had inherited from their father, something deeply rooted in their subconscious, but when such sentiments surfaced they became very conscious of them as such. The cause of their excesses was the extra amount of fire in the basic composition of their respective bodies.This genetic imbalance was responsible for such deeds as their act of violence against Joseph.

3The word מכרותיהם also contains the root מכר, sale. Jacob referred to the sale of Joseph as an act of violence perpetrated by these two brothers, seeing that Joseph had not been guilty. Although all the brothers had participated in that sale, closer examination of what happened will show you that these two brothers were the instigators of all that violence. They were the ones who said to each other in 37,19: "let us see what will become of his dreams when we will set out to kill him, etc." (compare Bereshit Rabbah 99,6).