The Gemara relates: One day, Rabbi Yoḥanan was bathing in the Jordan River. Reish Lakish saw him and jumped into the Jordan, pursuing him. At that time, Reish Lakish was the leader of a band of marauders. Rabbi Yoḥanan said to Reish Lakish: "Your strength is fit for Torah study." Reish Lakish said to him: "Your beauty is fit for women." Rabbi Yoḥanan said to him: "If you return to the pursuit of Torah, I will give you my sister in marriage, who is more beautiful than I am." Reish Lakish accepted upon himself to study Torah. Subsequently, Reish Lakish wanted to jump back out of the river to bring back his clothes, but he was unable to return, as he had lost his physical strength as soon as he accepted the responsibility to study Torah upon himself.
ריש לקיש זבין נפשיה ללודאי שקל בהדיה חייתא וגלגלתא אמר גמירי דיומא בתרא כל דבעי מינייהו עבדי ליה כי היכי דליחול אדמיה יומא בתרא אמרו ליה מאי ניחא לך אמר להו בעינא אקמטינכו ואותבינכו וכל חד מינייכו אמחי' חייתא ופלגא קמטינהו ואותבינהו כל חד מינייהו כד מחייה חד חייתא נפק נשמתיה חרקיניה לשיניה א"ל אחוכי קא מחייכת בי אכתי פש לך גבי פלגא דחייתא קטלינהו כולהו
The Gemara recounts a related incident: Reish Lakish sold himself to gladiators. He took a bag and a round stone inside of it with him. He said: There is a tradition that on the final day of a captive’s life, before his captors kill him, they do for him anything that he requests of them, so that he would forgive them for the spilling of his blood. On the final day before they were set to kill him they said to him: What is amenable to you? He said to them: I want to tie you up and have you sit, and I will strike each one of you one and a half times. He tied them up and had each one of them sit. When he struck each of them with one strike with the stone in the bag, the one whom he struck died, because Reish Lakish was of great strength. Reish Lakish gritted his teeth in anger, and said to the one whom he killed, in order to prevent the others from realizing what was happening: Are you laughing at me? You still have half of a strike remaining with me, as I struck you only once. He killed them all, and Reish Lakish escaped his captors.
...Doesn’t Ulla say: "When one sees Reish Lakish studying Torah in the study hall it is as though he is uprooting mountains and grinding them into each other?"
Reish Lakish said: Great is repentance, as one's intentional sins are counted for him as unwitting transgressions, as it is stated: “Return, Israel, to the Lord your God, for you have stumbled in your iniquity” (Hosea 14:2). The Gemara analyzes this: Doesn’t “iniquity” mean an intentional sin? Yet the prophet calls it stumbling, implying that one who repents is considered as though he only stumbled accidentally in his transgression. The Gemara asks: Is that so? Didn’t Reish Lakish himself say: Great is repentance, as one’s intentional sins are counted for him as merits, as it is stated: “And when the wicked turns from his wickedness, and does that which is lawful and right, he shall live thereby” (Ezekiel 33:19), and all his deeds, even his transgressions, will become praiseworthy? The Gemara reconciles: This is not difficult: Here, when one repents out of love, his sins become like merits; there, when one repents out of fear, his sins are counted as unwitting transgressions.
Rabbi Yoḥanan taught Reish Lakish Bible, and taught him Mishna, and turned him into a great man. Eventually, Reish Lakish became one of the outstanding Torah scholars of his generation. One day the Sages of the study hall were engaging in a dispute concerning the following baraita: With regard to the sword, the knife, the dagger, the spear, a hand sickle, and a harvest sickle, from when are they susceptible to ritual impurity? The baraita answers: It is from the time of the completion of their manufacture, which is the halakha with regard to metal vessels in general. These Sages inquired: And when is the completion of their manufacture? Rabbi Yoḥanan says: It is from when one fires these items in the furnace. Reish Lakish said: It is from when one scours them in water, after they have been fired in the furnace. Rabbi Yoḥanan said to Reish Lakish: A bandit knows about his banditry, i.e., you are an expert in weaponry because you were a bandit in your youth. Reish Lakish said to Rabbi Yoḥanan: What benefit did you provide me by bringing me close to Torah? There, among the bandits, they called me: Master ("rabbi"), and here, too, they call me: Master. Rabbi Yoḥanan said to him: I provided benefit to you, as I brought you close to God, under the wings of the Divine Presence.
More sources for future shiurim
Further evidence that R"L had an inner struggle with his past going on:
More evidence of R"L's dedication to study: