Rashi (citing Targum Onkelos): Because, since he was speaking about David, ¹ he refers to him first at the beginning of his leadership (still in the days of Sha'ul), ² and then, to the period after he was crowned king, when he would establish a firm grip on the throne and (like a lion and lioness in their lair), he would be invincible.

Targum Yonasan: First, he refers to the two sinful episodes ³ that had already passed - for which Yehudah himself had already attained atonement - then, to his leadership as a king.

Rashbam: He compares Yehudah to a lion cub, because it is both swifter and stronger than an old lion; he then compares him to a lion and a lioness, referring to after he was established and sitting safely on the throne, when the enemy no longer dared challenge him.

Seforno: He refers to Yehudah as a lion cub, since, although he did not yet have a kingdom, he ruled over his brothers, and was destined for sovereignty. He did not order Yosef's death ⁴ because he was like a lion crouching peacefully, as lions who caught their prey tend to do, and who do not arise to flee before any other wild animals that appear.

Ohr ha'Chayim: Adam ha'Rishon was like a tree, on which hung all Neshamos of Kedushah that will descend into the world. When he sinned, evil ruled, and innumerable [of those Neshamos] were taken captive. Yisrael strive to recover them via the Torah. Or, it can be via clinging to Tum'ah, b'Ones. "Va'Tidbak Nafsho b'Dinah" (34:3) - Dinah's Kadosh Nefesh drew that of R. Chanina ben Teradyon, which was in Shechem; "RaChaVaS Yadayim" (34:21) hints to this. Sometimes the Neshamah itself seeks the good path, and converts. The Neshamah can be captive in the lowest person; Rus and Na'amah come from Zenus of Lot with his daughters! Hashem forced Yehudah to be with Tamar - He hid from him that it is his daughter-in-law - to bring out the captive Neshamos of Peretz and Zerach. Only Yehudah, who has the aspect of kingship, could do so. Before Ma'aseh Tamar, Yehudah was a lion cub; his greatness would not last for generations. Through that episode, he grew [and eternal kingship comes from him].

Malbim: He is strong like an adult lion, and zealous and quick in his matters like a cub.

Ha'amek Davar: Also tigers, bears and other animals tear (kill). Only a lion, when it sees a person bearing Tzelem Elokim, it turns back.

Maharal (Gevuros Hashem Ch. 15, p. 74): Animals move more swiftly than man; this is due to their light Chomer (material composition). Yisrael also is only slightly material (for they are primarily spiritual). ¹

Rashi: From the killing of Yosef - of which Yaakov had initially suspected him; ¹ and from the killing of Tamar, when he admitted and declared "Tzadkah Mimeni!" (38:26).

Rashbam: He was referring to after David had captured the nations and settled down in Yerushalayim. ²

Hadar Zekenim: Hashem saved David from a lion and a bear. Due to this, he fought Golyas and rose.

Riva: It was [only] from killing Tamar.

Ohr ha'Chayim: From the Kadosh Neshamos captive amidst evil (refer to 49:9:1:5), you were elevated. Your own Neshamah was fit only for kingship for itself; now, eternal kingship comes from you!

Ha'amek Davar: Even when you wanted to kill a man, when you saw that he is "Beni" (a person with attributes), you overcame your desire.

Rashi #1: Refer to 49:9:1:1.

Targum Yonasan: Refer to 49:9:1:2.

Rashi #2: It was because Yehudah displayed that great strength of character (refer to 49:9:2:1) that he earned the merit for his descendant Shlomo to dwell peacefully in Eretz Yisrael, ¹ like a lion and lioness in their lair. ²

Ohr ha'Chayim: Because Yehudah was like a lion of Kedushah, he was able to take the Kadosh Neshamos that had been captive amidst evil (refer to 49:9:1:5).

Ohr ha'Chayim: That prey (Kadosh Neshamos captive amidst evil; refer to 49:9:1:5) crouched upon you - "la'Pesach Chatas Rovetz" (4:7).

Malbim: A male lion bends on its knees; a female with cubs, it crouches. No one dares to make it rise! So too, no warrior will approach you.

Ha'amek Davar: There are stadium lions, which their overseers prod to make a kill. However, when a person bearing Tzelem Elokim is in front of it, it crouches. Even if its overseer tries to make it rise to kill the person, it does not budge. So, if the ruler in Yehudah agrees to have mercy on Beni, even if his advisors urge him to kill him, no one can persuade him. This refers to David, who refused to kill Sha'ul and Shim'i ben Gera, due to their attributes. His men, Avishai and his other counselors could not persuade David to harm them. ¹