וַיֹּ֤אמֶר אַבְנֵר֙ אֶל־יוֹאָ֔ב יָק֤וּמוּ נָא֙ הַנְּעָרִ֔ים וִֽישַׂחֲק֖וּ לְפָנֵ֑ינוּ וַיֹּ֥אמֶר יוֹאָ֖ב יָקֻֽמוּ׃
Então disse Abner a Joabe: Levantem-se os mancebos, e se batam diante de nós. Respondeu Joabe: Levantem-se.
Rashbam
את אחיו, he was tending the sheep with his brothers, but, seeing that he still enjoyed his carefree childhood he spent most of his time with the sons of Bilhah and Zilpah, instead of in the company of the sons of Leah. As a 17 year old he should have acted like a נער, an adolescent lad approaching manhood. This is the meaning of the word נער in Hoseah 11,1 Samuel II 2,14. The Torah now enumerates a number of other causes which contributed to the brothers hating Joseph. [this was how Leah’s sons viewed his behaviour, not how Joseph explained it. Ed.]
Rashi
לצחק TO PLAY — There is implied in this term besides idolatry also sexual immorality, — as we find the word used in, (Genesis 39:17) “to mock (לצחק) me” where unchastity is meant as is evident from the context — and blood-shed, as it is said, (II Samuel 2:14) “Let the young men arise and play (וישחקו) before me; [and they caught every one his fellow by the head and thrust his sword in his fellows side]”— here, too, Hur was assassinated (Midrash Tanchuma 3:9:20).
Rashi
מצחק MAKING SPORT — This means worshipping idols, as it is said in reference of the Golden Calf, (Exodus 32:6) “And they rose up to make merry (לצחק).” Another explanation is that it refers to immoral conduct, just as you say in reference to Potiphar’s wife, (Genesis 39:17) “To mock (לצחק) at me.” Another explanation is that it refers to murder, as (2 Samuel 2:14) “Let the young men, I pray thee, arise and make sport (וישחקו) before us” (where they fought with and killed one another) From Sarah’s reply — “for the son of this bondwoman shall not be heir with my son״ — you may infer that he (Ishmael) was quarrelling with Isaac about the inheritance, saying, “I am the first-born and will, therefore, take a double portion”. They went into the field and he (Ishmael) took his bow and shot arrows at him (Isaac), just as you say (Proverbs 26:18-19) “As a madman who casteth firebrands, [arrows and death] and says: I am only מצחק mocking” (Genesis Rabbah 53:11).
Rashi
If you had not spoken. If only you had spoken this [way5Your decrying of us fighting each other. earlier] [The לא] is as [if it were written] לו6According to this first interpretation, the construct לוּלֵא denotes לוֹלוּ, “if only if,” i.e., if only you had spoken these reconcilitory words earlier., meaning it. Another [way] to interpret it is as it's plain meaning7This interpretation of לולא is the usual “if not for.”: if you had not spoken i.e. that which you said "Let the young soldiers get up and playfight."8Those words triggered this deadly battle.